Infectious Respiratory Disease

Content

This unit will span approximately three to six weeks and will fall within the broader themes of health science, biology to include the structure/function of body systems, cells, and disease prevention. The purpose is to help students understand the science behind how the flu spreads, how our bodies defend against it, and how public health efforts work to control outbreaks. This will not only support science literacy but also promote critical thinking, empathy, and responsible behavior. For this unit, the following key learning objectives and topics have been identified to get the information across. These are observable and measurable, and they align with both the Next Generation Science Standards (NGSS) and the real-world application of health science. Each objective will be assessed through student discussions, lab reflections, exit tickets, quizzes, and a culminating project that showcases both content knowledge and communication skills.

By the end of the unit, students will be able to:

1. Identify and describe the main routes of influenza transmission (e.g., airborne droplets, contact with contaminated surfaces).
2. Explain how the human immune system responds to a viral infection like influenza.
3. Evaluate personal and community-based strategies used to control flu outbreaks, including vaccinations, hygiene, and public health protocols.
4. Create a public health campaign (poster, video, or digital product) that communicates prevention strategies effectively.

Key Topics

1. What is influenza? How it spreads (droplet, contact, and airborne transmission) and understanding viruses vs. bacteria
2. Common flu symptoms and how the virus affects the body
3. The body’s defense: the immune response explained
4. Prevention strategies: vaccines, hand hygiene, masking, and distancing
5. Public health measures: quarantine, health education, and global responses
6. Reflecting on past pandemics (e.g., 1918 Spanish Flu and 2009 H1N1) and comparing flu and COVID-19: what we’ve learned.

Overview of Influenza

The learning objectives and topics connect to the overall goal of the unit as it aims to empower students to understand how diseases like influenza are transmitted and what can be done to reduce their spread, both individually and as a community. The learning objectives are closely aligned with this goal, as each one helps build toward a broader understanding of health, responsibility, and prevention.

Students will explore the concept that respiratory infectious disease usually starts with a slight cough, a sore throat, sometimes a little sniffle, and we generally think nothing of it. The frequent utterances are “this allergy” or “my sinuses,” and other times, “it's just this chilly morning air.” They will understand that influenza has a very short incubation period and the symptoms do not evolve slowly; it hits you like a truck.  The fatigue, the repeated sneezing, and uncontrollable shivering can be even more devastating for some individuals. Then, before you know it, everyone who was around you the previous day is experiencing similar symptoms. A visit to the doctor reveals that you have the dreaded, sneaky, fast-spreading virus, influenza. These are a few of the common early symptoms of the flu that affect a person’s nose, throat, and lungs. This life-threatening illness remains a major public health concern globally because of its ability to cause annual epidemics and worldwide pandemics. The good thing about influenza is that we are fortunate to know how it spreads and how to control it.³

Influenza, commonly referred to as the flu, is an acute respiratory illness caused by the influenza RNA viruses, “flu viruses,” which circulate worldwide and cause annual epidemics, resulting in about 3–5 million cases of severe illness and 290,000–650,000 deaths annually. This highly infectious disease not only affects humans but can also infect animals. World Health Organization (WHO), 2018. In addition to the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO), several peer-reviewed sources highlight that there are four types of influenza viruses according to the combination of proteins. These contagious pathogenic microorganisms belong to the Orthomyxoviridae family, which consists of four species and is grouped according to the combination of proteins on the surface of the virus: influenza A virus (IAV), influenza B virus (IBV), influenza C virus (ICV), and influenza D virus (IDV).⁴

Types of Influenza Viruses

The influenza A virus (IAV) is quite severe and sometimes causes death. It affects humans, birds, pigs, and other animals. It mutates frequently and causes seasonal epidemics and pandemics. Scientists classify it into two subtypes called A(H1N1) and A(H3N2) influenza viruses. The A(H1N1) is also written as A(H1N1) pdm09, as it caused the pandemic in 2009 and replaced the previous A(H1N1) virus, which had circulated before 2009. Influenza B viruses that affect humans only are not classified into subtypes but can be broken down into two main genetic lineages referred to as B/Yamagata or B/Victoria lineage.⁵ These names come from the geographic locations where each lineage was first identified in Yamagata, Japan, and Victoria, Australia. Influenza B is typically less severe than type A, causes seasonal epidemics but not pandemics. Influenza C virus is detected less frequently and usually causes mild infections, and does not present any major public health concerns as it does not cause human epidemics.⁶ Influenza D viruses primarily affect cattle, pigs and are associated with bovine respiratory disease in livestock. It is not known to infect or cause illness in people.⁷

Viruses are non-living, but they are able to infect living cells and replicate. These intracellular parasites adapt to their specific host organism or cells and are unable to reproduce through division, unlike other microscopic organisms, such as bacteria or yeast. They can re-route and modify the course of cellular processes for the optimal execution of their own reproduction. Apart from the influenza viruses, there are many different types of viruses, such as herpes simplex viruses, varicella-zoster virus, and bovine and human papillomaviruses. A virus exists in different conditions and behaves differently in its host cells. The general life cycle of a virus includes the following stages: attachment, entry, uncoating, replication, assembly, and release.⁸

Signs and Symptoms of Influenza

The signs and symptoms of Influenza are many and varied, and can also range from mild to severe. The severity of the symptoms also varies from person to person, depending on whether a person is immunocompromised or if they are predisposed. Factors such as vaccination status, age, and nutritional status play an important role in how severe the symptoms can be.  Common symptoms of the flu can include fever or feeling feverish/chills, cough, sore throat, runny(rhinitis) or stuffy nose, muscle or body aches (myalgia), general discomfort (malaise), headaches, and fatigue (tiredness).⁹ Children sometimes experience nausea, vomiting, or diarrhea. Most uncomplicated cases resolve within 3–7 days, but cough and fatigue can linger for over two weeks, especially in older adults. Fever may be absent in older adults or infants. Students must be aware of the fact that serious complications can arise from influenza and should never be taken lightly. Complications may include pneumonia, bronchitis, sinus infections, or worsening of pre-existing health issues.¹⁰

There are many similarities and differences between influenza and the common cold. Students need to understand that it can be difficult to differentiate between the two. Most importantly, the flu virus can mutate rapidly in a relatively short time and has the potential to be catastrophic, like the 1918 Spanish flu. According to the CDC, cold symptoms are usually milder and occur in the upper respiratory tract. Clinical tests are sometimes needed to distinguish between the two. The flu is an illness brought on by an influenza virus infection and can be deadly, while the common cold rarely is. The common cold is typically not afflicted by a foreign pathogen but rather represents a conflict between the body's defensive mechanisms and the typical flora of viruses that live in the upper respiratory system. Therefore, flu epidemics only happen during certain limited times, whereas the common cold occurs whenever people feel cold for a considerable amount of time throughout the year.¹¹

Immune System Response to Influenza

Your immune system starts working as soon as the flu virus enters your body, generally through your mouth or nose. Your immune system acts as a group of bodyguards to keep you healthy. The smart combatants and the quick responders make up its two primary components. The innate immune system is the initial line of protection. This is the rapid response team in your body.¹² The virus is first detected by cells in your throat and nose. They call for assistance by sending out specific chemicals. White blood cells, such as macrophages and natural killer cells, then arrive to combat the virus and eliminate any harmed cells. This response occurs quickly, but it lacks specificity. The adaptive immune system is the second component. It is more intelligent and potent, but it takes a little longer to start working.¹³Antibodies, which bind to the virus and prevent it from spreading, are produced by this mechanism. These antibodies are produced by B cells, which are unique cells. T cells are additional cells that aid by either destroying infected cells or increasing the antibody response. Your body doesn't forget once it has fought off the virus. It produces memory cells that will remember how to combat the same virus if it resurfaces. This implies that the next time you are exposed, your body will be able to react more quickly and powerfully.

Since the immune system is the body's defensive mechanism against pathogens like bacteria and viruses that can cause illness, we have to maintain its functionality. There are several ways we must care for our bodies. Eating a healthy diet is one of the most crucial strategies to boost immunity. Our immune cells require vitamins and minerals, which can be obtained by eating a balanced diet full of fruits, vegetables, whole grains, lean meats, and healthy fats. Particularly beneficial for increasing immunity are foods strong in zinc (found in beans, nuts, and seeds), vitamin D (found in eggs, salmon, and fortified milk), and vitamin C (found in oranges, strawberries, and bell peppers.¹⁴

A strong immune system also depends on getting enough sleep. Our bodies rest and heal themselves while we sleep. The majority of kids and teenagers require 8 to 10 hours of sleep every night. Our bodies create fewer cells that fight infections when we don't get enough sleep, which makes it more difficult to stay healthy. Immunity can be enhanced by frequent exercise in addition to sleep. Exercise facilitates the flow of immune cells throughout the body, improves blood circulation, and reduces stress. Simple activities like going for a walk, riding a bike, or participating in sports can have a significant impact.¹⁵

Maintaining proper hygiene practices, such as frequent hand washing, concealing coughs and sneezes, and avoiding ill people, can help your body fight off fewer bacteria. Additionally, stress management is crucial since chronically high levels of stress can impair immunity. Maintaining your physical strength can be achieved by finding healthy ways to unwind, such as reading, sketching, conversing with a trusted person, or going outside. Receiving your immunizations on time trains your immune system to identify and combat dangerous infections like the flu.¹⁶

History of Influenza

There have been many notable pandemics throughout history; however, the 1918 influenza pandemic was the most severe in recent times, caused by an H1N1 virus with genes of avian origin, and it spread worldwide with an estimated 500 million people infected and at least 50 million deaths, according to the Centers for Disease Control and Prevention (CDC). John M. Berry, in his book “The Great Influenza,” describes the 1918 influenza as the deadliest pandemic in history. It focuses on the science and the politics behind the approach to the virus. It also highlights the history of the virus and how it can serve as a lesson for the future. The 1918 influenza spread like wildfire and infected about one-third of the world’s population, and caused over 50 million deaths. At that time, vaccines and antiviral medications were not yet developed, hence people wore masks and avoided crowded areas to ensure their safety.¹⁷

In addition to the deadliest 1918 influenza outbreak known as the Spanish flu, there have been other large-scale disease outbreaks that have occurred frequently throughout history, though not consistently. For instance, the Asian flu struck in 1957, around 40 years later, and the Hong Kong flu struck in 1968, only 11 years later. Another flu, known as H1N1, or "swine flu," sickened a lot of people in 2009. In 2019, just ten years later, COVID-19 struck, which took the lives of over 7 million people worldwide according to the WHO. These pandemics occur when novel viruses spread quickly from person to person, and they are becoming more frequent, possibly as a result of increased travel and closer living quarters.¹⁸

Although scientists were better prepared by that time, the Hong Kong flu (1968) and the Asian flu (1957) emerged; these infections still spread rapidly. People were alerted by news and health messages, and vaccines were produced more rapidly. Though not as numerous as the Spanish flu, these pandemics nonetheless claimed many lives. The H1N1 virus expanded globally in 2009. Health officials used technology, such as websites and text alerts, to notify people, and scientists were able to produce a vaccine more quickly. Businesses and schools provided health advice or temporarily closed to assist. Conversations about the 1918 influenza are usually more poignant because it presented itself in three deadly waves.¹⁹

Scientists made better progress with the H1N1 flu (sometimes known as the "swine flu") in 2009. By then, they were able to produce a vaccine more quickly that significantly minimized its effect globally, and health officials informed the public using technology such as SMS alerts and by placing information on websites. Businesses and schools provided health advice or temporarily closed their doors to assist in lessening its spread. Individuals responded more positively when compared to the previous pandemics. Although COVID-19 was a different kind of virus from the flu, we applied many of the lessons learnt from previous pandemics, including mask use, hand washing, social distancing, and rapid vaccine development. Over time, previous flu pandemics have taught us valuable lessons. One of the most important lessons is that, when developed and administered promptly, vaccinations can save countless lives. We also know that early dissemination of health information keeps individuals safe and prevents the spread of disease. Easy ways to protect others include wearing masks, washing your hands frequently, and remaining at home when you're unwell. We now know that nations must cooperate during a pandemic since viruses can move globally and infect people everywhere.²⁰

Routes of Transmission

The influenza virus can be transmitted from one person to another in various ways. An understanding of these routes of transmission is crucial in preventing its spread since it has the potential to be transmitted quite easily. These well-established routes of transmission include airborne transmission (aerosols vs. droplets), contact transmission, and fomite transmission (Contaminated Surfaces).²¹

When an infected person coughs, talks, breathes, or sneezes, droplets do not remain suspended in the air for too long but will settle quickly on surfaces or fall to the ground, depending on how large they are. Smaller particles can linger much longer in the air before settling. They fall to the ground quickly and may only travel within 1 to 2 meters or 3 to 6 feet. Hence, the reason why physical distance is recommended. An individual can become infected if the influenza virus enters their mouth, nose, or eyes. Aerosols are tiny airborne respiratory particles that are smaller than 5 micrometers (µm) in diameter. Their size makes them relatively easy to travel farther distances and can be suspended in the air for minutes to hours, especially in poorly ventilated or crowded spaces. This also allows them to travel deeper into the lungs with much ease. Environmental factors like low humidity and cool temperatures can enhance aerosols’ survival and longevity.²²

Contact transmission refers to the ability of the virus to spread through direct physical contact with an infected person. This may occur through hugging, kissing, a handshake, or touching a person’s face, as this can introduce the virus to their respiratory system. Having a knowledge of this mode of transmission will enable students to see the importance of proper hygiene practices, such as washing hands thoroughly with soap and water or using an alcohol-based hand sanitizer, and avoid touching each other when sick. Studies have shown that the virus can survive on hands for between 5 to 15 minutes. This is a significant amount of time for an individual whose hands are contaminated to spread the virus to others or infect themselves with the virus by touching their face.

Fomite Transmission refers to how objects or surfaces can become contaminated with the virus and can serve as a vehicle for the spreading of the disease. Studies show that fomites play a significant role in the spread of the virus. Examples of common fomites that contribute to the spread of the influenza virus are: doorknobs, phones, handrails, money, and computer keyboards. The virus can survive on these surfaces for a long time after an infected person coughs, sneezes, or touches these objects. Another person then allows the virus to be introduced to his or her body through the nose, mouth, and eyes after touching these contaminated surfaces.²³

There are a few factors that influence the spread of the influenza virus. These include humidity, temperature, ventilation, crowding, hygiene, and seasonal patterns can also cause its spread dramatically. Thousands of people die from the flu yearly; however, the 2021-2022 flu season saw a significant decline in deaths due to covid prevention strategies, in addition to many of these factors. Low temperatures (≈5°C) and low relative humidity (20–35%) greatly increase influenza transmission, whereas higher humidity (80%) or warmer temperatures (30°C) prevent the spread, according to guinea pig studies. According to environmental assessments, winter outbreaks are encouraged by cool, dry weather because it preserves virus particles and weakens hosts' mucosal defenses. Poor hygiene and higher person-to-person contact can significantly facilitate its spread.²⁴

Control and Preventive Measures

Students need to be aware that there are measures that can be taken to mitigate or prevent influenza transmission, as acquiring this highly contagious disease can seriously affect them and can also be fatal. According to the CDC, some of these strategies include vaccination, antiviral medications, promoting good hygiene such as proper hand washing, social distancing, and staying home when infected, wearing masks, and environmental controls such as ventilation and disinfecting.²⁵ These simple measures can significantly reduce the transmission of the virus or minimize its effects after contracting the virus.

Vaccination

One of the best ways to reduce the effects and spread of seasonal influenza is by vaccination. The flu vaccine functions by inducing the production of antibodies by the immune system, mainly against the influenza virus's hemagglutinin (HA) protein. If the virus is later encountered, these antibodies allow the body to identify and neutralize it. Influenza vaccinations come in two primary varieties: live attenuated vaccines, like the nasal spray, which use a weakened strain of the virus that replicates in the nasal passages without producing sickness, and inactivated vaccines, which are administered by injection and include viruses that have been killed.²⁶

Each year, influenza vaccines are reformulated to match the most prevalent strains predicted to circulate during the upcoming flu season. This is necessary because influenza viruses frequently mutate through a process known as antigenic drift. The World Health Organization (WHO), in coordination with national health agencies like the Centers for Disease Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA), selects the specific virus strains to include in the seasonal vaccine.²⁷ Components of U.S. vaccinations for the 2024–2025 season target one influenza B lineage and two influenza A subtypes (H1N1 and H3N2).²⁸

Those who have had vaccinations have a lower risk of problems, hospitalization, and critical care needs. For vulnerable groups, including children under five, adults over 65, pregnant women, and people with long-term medical issues, the vaccine is especially important.²⁹  Additionally, pregnant women give their unborn children antibodies, which provide protection in the first few months of life when the child is too small to receive a vaccination.³⁰

In addition to protecting individuals, widespread vaccination also helps to build community immunity, or herd protection, which lowers the spread of viruses and protects those who are unable to get the shot for medical reasons. But even with these advantages, the U.S. flu vaccine rate is still below ideal. Adult immunization rates for the 2023–2024 season were approximately 45%, which was below public health goals.³¹ There are still differences between some racial, ethnic, and socioeconomic groupings. Public health professionals advise that evidence-based tactics include provider reminders, school-based and workplace vaccination programs, and community-specific outreach initiatives that are sensitive to cultural differences in order to address this.³²

In conclusion, vaccination is still the most effective way to prevent influenza. It lowers disease, lessens serious consequences, safeguards vulnerable groups, and aids in stopping the spread of infection among the general public.³³ To increase vaccination uptake and attain wider protection against influenza each year, persistent public health initiatives are necessary.

Antiviral Medication

Together with immunization, antiviral drugs are an essential part of all-encompassing influenza prevention and control plans. These drugs are especially crucial for those who are already experiencing symptoms of the flu or who are at a high risk of developing significant consequences from it. When used promptly, they can dramatically shorten the length of the disease, lessen its severity, and lower the risk of hospitalization and death. According to CDC clinical guidelines, antiviral medications work best when taken within 48 hours of the onset of symptoms; however, hospitalized or high-risk individuals may still benefit from later treatment.³⁴

There is proof that early antiviral therapy has noticeable therapeutic advantages. According to observational studies and meta-analyses, flu antivirals lower the risk of complications, including pneumonia and intensive care unit hospitalization, and shorten the duration of symptoms by around one day. According to one CDC study that included more than 26,000 hospitalized individuals, 30-day mortality was lowered by 40% when treatment was started on the day of admission as opposed to several days later. CDC, “CDC Study Shows Early Flu Antiviral Treatment Decreases Risk of Death.³⁵ Antiviral medication use has decreased recently, highlighting the need to strengthen treatment standards, yet it has been linked to lower rates of hospitalization and life-threatening outcomes among children and adolescents.³⁶

Beyond their advantages, antivirals support public health by providing a crucial barrier of defense for susceptible or unvaccinated groups. Antiviral resistance must be taken into account while weighing the use of antivirals. Influenza viruses can change, which makes medications less effective. CDC surveillance programs track changes in viruses, and because of widespread resistance, antivirals such as amantadine are no longer advised. Even though resistance cases are still rare, careful prescription practices and continuous monitoring are crucial to maintaining treatment effectiveness.³⁷

Public Health Strategies

Effective seasonal influenza control goes well beyond personal immunization and care. In order to prevent the spread of viruses in communities, public health organizations employ a variety of non-pharmaceutical interventions (NPIs), such as respiratory etiquette, hand cleanliness, urging sick people to stay at home, and promoting cleaner air. Schools can optimize ventilation and maintain improvements to indoor air quality to reduce the risk of germs and contaminants spreading through the air. These easy, daily actions are an essential line of defense because they are remarkably efficient in preventing spread, particularly during times of high activity.³⁸

More focused infection prevention and control strategies are essential in hospital environments. According to guidelines, symptomatic patients should be given masks when they arrive, individuals suspected of having the flu should be isolated, and facility-wide procedures, including easy access to facilities that promote proper hand hygiene and a waiting area that encourages spatial separation. By taking these precautions, healthcare capacity is maintained during surges, and nosocomial transmission between patients and staff is decreased.^{39 40}

Early detection and surveillance are also key components of public health programs. The WHO's Global Influenza Surveillance and Response System (GISRS) is one example of a globally coordinated system that tracks circulating strains and provides information for the yearly vaccine formulation. Health authorities can identify outbreaks early, direct timely public messaging, and administer vaccines or antivirals when necessary thanks to real-time surveillance, which includes laboratory reporting, electronic health records, and near-real-time tools.⁴¹ Home testing kits are also available, similar to the COVID-19 kits.

Community mitigation measures like social distancing, voluntary isolation or quarantine, school closures, and mask use in public can postpone and lessen peak transmission during influenza pandemics and periodic epidemics. Previously, these measures were only used during pandemics, but COVID-19 lessons have demonstrated that combining NPIs, such as masking, distancing, and better ventilation, greatly reduces influenza incidence and relieves strain on helth systems.⁴²

Lastly, prevention is greatly aided by workplaces and public institutions. Employers are urged to provide flu shots on-site, encourage sick leave, and enforce hand cleanliness and cough etiquette.⁴³⁴⁴ Employer-sponsored flu preventive programs are especially beneficial for healthcare, daycare, and agricultural workers because these environments often increase the spread of the virus if they are not properly managed.⁴⁵

Impact of Influenza on Daily Lives & School Absenteeism

Daily routines can be severely disrupted by influenza, especially for families with school-age children. Absences due to illness are common during flu season. Flu outbreaks cause significant absenteeism rates in schools every year. According to a CDC survey, schools close when absenteeism reaches 15% or higher, which has repercussions like parents missing work.⁴⁶ According to another study, opening in the fall frequently corresponds with discernible increases in influenza transmission, underscoring the impact of school calendars on community spread.⁴⁷

Children are particularly susceptible to catching and transmitting the influenza virus. Close contact behaviors, such as sharing toys, eating meals together, and neglecting to wash one's hands properly, facilitate the spread of viruses in instructional settings. The flu virus can survive on surfaces for up to 48 hours, making contaminated desks or doorknobs indirect transmission vectors. Children may take the infections home before they show symptoms, which increases the risk to vulnerable friends and family members in addition to causing interruptions like missed class time, which can significantly affect their learning.⁴⁸