Influenza & Viral Respiratory Illness
Flu High-yield Verified · Sep 2026Influenza is a contagious respiratory illness caused by influenza viruses that infect the nose, throat and sometimes the lungs; it ranges from mild to severe illness and at times causes death.
Sudden-onset fever and aches, the 48-hour antiviral window and who is treated anyway, droplet precautions for seven days, the high-risk list, the warning signs that send people to care, and the vaccine myths nurses still have to correct.
What’s going wrong in the body
The normal physiology, where it fails, and why the body’s compensation backfires.
01 A virus that changes every season
Influenza A and B cause the seasonal epidemics — in the Northern Hemisphere, typically between October and March. Influenza A is sub-typed by the two proteins on its surface, hemagglutinin (H) and neuraminidase (N); the current seasonal A strains are A/H1N1 and A/H3N2. Influenza A also lives in birds and many other animals, which is where new strains such as avian A(H5N1) come from.
Drift and shift explain the yearly vaccine and the pandemics. The viruses change constantly by antigenic drift — "they can change from one season to the next or they can even change within the course of one flu season" — so vaccine composition is reviewed every year. Antigenic shift is the other way the virus changes, and a pandemic follows when a new influenza A virus appears: "A flu pandemic is a global outbreak of a new influenza A virus."
It is common and serious. About 8% of the US population gets sick with flu each season (range 3% to 11%), and an estimated 100,000 to 710,000 people are hospitalized each year for influenza-related complications.
02 Attach, replicate, release — and strip the airway bare
Two surface proteins do the work. "Hemagglutinin adheres to the epithelial cells in the respiratory tract allowing for the progression of the infection. Neuraminidase cleaves the bond that holds the virus together and helps to spread the virions." Neuraminidase is the step the antivirals oseltamivir, zanamivir and peramivir block — they stop newly made virus leaving the cell.
The clock runs fast. Incubation is about two days (range one to four), and replication in the upper and lower airways peaks after about 48 hours on average. That is the physiological reason treatment works best early: an antiviral that stops release helps most while the virus is still multiplying.
The symptoms are the immune response. "The immune reaction to the viral infection and the interferon response are responsible for the viral syndrome which includes high fever, coryza, and body aches." Meanwhile the virus destroys the lining of the trachea and bronchi, leaving only the basal layer — the ciliated defence that normally clears bacteria is gone, which is why secondary bacterial pneumonia follows.
It can escalate within two days. Flu can trigger "an extreme inflammatory response in the body and can lead to sepsis," and severe cases can progress to shortness of breath, tachycardia, hypotension and the need for respiratory support "in as little as 48 hours" from the start of symptoms.
03 Contagious before symptoms, and for about a week
Mainly droplets. "Most experts believe that flu viruses spread mainly by droplets made when people with flu cough, sneeze, or talk." Large droplets travel only short distances — approximately 6 feet or less — so transmission needs close contact. Less often, people catch it by touching a contaminated surface and then their mouth, nose or eyes.
The contagious window starts early. People can spread flu before they know they are sick; they are most contagious in the first three days, and some otherwise healthy adults can infect others from one day before symptoms to five to seven days after becoming sick. Young children and people with weakened immune systems may be contagious for longer. In a 2017–2023 household study, 8% of people who tested positive had no symptoms at all.
This is where the hospital rule comes from. CDC’s Droplet Precautions for influenza last "for 7 days after illness onset or until 24 hours after the resolution of fever and respiratory symptoms, whichever is longer" — a duration built around the shedding period, extended further for young children and the severely immunocompromised.
04 Who gets an antiviral — the 48-hour window is not a cut-off
CDC’s priority groups are treated no matter what. Antiviral treatment is recommended as soon as possible for anyone with suspected or confirmed influenza who is hospitalized, has severe, complicated or progressive illness, or is at higher risk for complications — "Empiric antiviral treatment should be started as soon as possible in the above priority groups," without waiting for a test result.
The 48 hours is about size of benefit. Treatment is best "ideally within 48 hours of symptom onset for the greatest clinical benefit," but observational studies show benefit in severe, complicated or progressive illness and in hospitalized patients when started later. For people not at increased risk, most do not need antivirals; clinicians can consider them if treatment can start within two days of onset.
Choosing the drug. Hospitalized patients get oral or enteric oseltamivir; inhaled zanamivir, oral baloxavir and IV peramivir are not routine in hospital. Pregnancy also means oseltamivir — baloxavir is not recommended in pregnancy or while breastfeeding. The older adamantanes (amantadine, rimantadine) are not used because resistance is high.
Causes & risk factors
Modifiable factors are teaching targets — the patient can act on them.
Signs & symptoms
Each finding tied to the mechanism that produces it — and the triage it calls for.
Diagnostics
Complications
Medications for this condition
Why each class is prescribed here — every card links to its full pharmacology page.
Recommended as soon as possible for anyone with suspected or confirmed flu who is hospitalized, has severe, complicated or progressive illness, or is at higher risk for complications — empirically, without waiting for a test. Best "ideally within 48 hours of symptom onset for the greatest clinical benefit," but still beneficial later in hospitalized, severe or progressive illness. Hospitalized patients and pregnant patients get oral oseltamivir; the course for uncomplicated flu is oseltamivir or inhaled zanamivir twice daily for 5 days, or a single dose of IV peramivir or oral baloxavir. Most healthy people with mild flu do not need them; when used, they shorten illness by about a day. Amantadine and rimantadine are not recommended because of resistance.
The main prevention. Everyone 6 months and older, with rare exceptions, every season; antibodies take about two weeks to develop. For people 65 and older, high-dose, recombinant or adjuvanted vaccines are preferentially recommended. Even when vaccinated people still get sick, vaccination was linked to 26% lower risk of ICU admission and 31% lower risk of death from flu.
The fever-reducing medicine CDC names — with the rule that return to work (or activities) is judged on being fever-free without fever-reducing medicine.
Beyond medications
- Home care for uncomplicated flu — stay home and away from others except to get care; hand washing; drinking ample fluids. Return to normal activities when, for at least 24 hours, symptoms are improving overall and there has been no fever without fever-reducing medicine.
- Everyday prevention — stay away from people who are sick, cover coughs and sneezes, wash hands often, and take steps for cleaner air.
- Antibiotics do not treat flu — "Antibiotics fight against bacterial infections and do not work against viruses, like influenza"; they are for a secondary bacterial pneumonia, not the virus.
- Vaccine timing — ideally September or October; keep vaccinating throughout the season while viruses circulate and unexpired vaccine is available.
- Chemoprophylaxis — an antiviral for 7 days after the last known exposure; in institutional outbreaks, for at least 2 weeks and until 1 week after the last case.
Nursing considerations
The RN-specific layer — each action paired with the reason it matters.
How a real nurse thinks
The clinical-judgment loop, worked for a typical presentation — reasoning, not reference.
- 1 Recognize cues
Cues: age 72 with COPD and diabetes (higher-risk), fever and cough that improved then returned and worsened, new purulent sputum, RR 28, SpO₂ 89%, HR 112, a negative rapid antigen test, and symptoms beyond 48 hours.
- 2 Analyze cues
Improve-then-worsen is a CDC warning sign and the classic pattern of secondary bacterial pneumonia on flu-damaged airways. A negative rapid antigen test does not rule flu out (low sensitivity), and vaccination does not exclude it. As a hospitalized, higher-risk patient she needs antivirals even after 48 hours.
- 3 Prioritize
Breathing first — hypoxia, tachypnea and tachycardia with a possible bacterial pneumonia is an escalation now. Then droplet precautions for staff and other patients, and the antiviral without delay.
- 4 Generate solutions
Apply oxygen as ordered and escalate the warning sign; get the chest X-ray done promptly (pulmonary symptoms, rule out bacterial pneumonia); start oseltamivir with food; droplet precautions and a private room; watch glucose and hydration; explain the treatment rationale to the family.
- 5 Take action
She calls the provider with an SBAR: 72-year-old with COPD and diabetes, flu symptoms day 3, fever and cough improved then worsened today with new yellow sputum, RR 28, SpO₂ 89% on 2 L, HR 112 — concerned about secondary bacterial pneumonia; chest X-ray ordered; requesting evaluation. She gives the first oseltamivir dose with a snack, posts droplet signage, and explains to the family that hospitalized and higher-risk patients are treated even after 48 hours and that rapid antigen tests often miss flu.
- 6 Evaluate
Improvement looks like falling temperature and heart rate, SpO₂ recovering on the ordered oxygen, and stable glucose. Droplet precautions continue for 7 days from onset or 24 hours after fever and respiratory symptoms resolve, whichever is longer; confusion, not urinating or rising oxygen needs re-escalate.
Scenario — you’re the nurse
Work each question before revealing the answer. Rationales explain the wrong options too.
A 72-year-old woman with COPD and type 2 diabetes admitted with 3 days of fever, cough and body aches. A rapid antigen test in the ED was negative; a molecular test is pending. She felt a little better yesterday, but today her fever returned and she is more short of breath. Orders: oral oseltamivir twice daily, droplet precautions, chest X-ray, blood glucose monitoring.
Q1 Select all that apply Which findings are CDC emergency warning signs in this patient? Select all that apply. - A. Fever and cough that improved, then returned and worsened
- B. Body aches on day 3
- C. Difficulty breathing with SpO₂ 89%
- D. Sore throat
- E. A negative rapid antigen test
Reveal answer & rationale
Q2 Multiple choice The family asks why oseltamivir is ordered when the rapid test was negative and symptoms began more than 48 hours ago. The best response is: - A. The provider must have made an error — antivirals only work within 48 hours
- B. Rapid antigen tests often miss flu, and hospitalized or higher-risk patients benefit from antivirals even after 48 hours
- C. Oseltamivir is given to prevent bacterial pneumonia
- D. The vaccine did not work, so she needs a second vaccine dose instead
Reveal answer & rationale
Q3 Multiple choice Which isolation is correct for routine care of this patient? - A. Airborne precautions in a negative-pressure room for all care
- B. Droplet precautions with a facemask on entry
- C. Contact precautions only
- D. Standard precautions only, because she was vaccinated
Reveal answer & rationale
Q4 Multiple choice How long should droplet precautions continue? - A. Until the first dose of oseltamivir is given
- B. 24 hours after admission
- C. 7 days after illness onset or 24 hours after fever and respiratory symptoms resolve, whichever is longer
- D. Until discharge, however long the stay
Reveal answer & rationale
Q5 Select all that apply Which instructions about her antiviral are correct? Select all that apply. - A. Take oseltamivir with food to lessen nausea
- B. Report new confusion or unusual behaviour
- C. Inhaled zanamivir would be a good alternative because of her inhaler experience
- D. The usual treatment course is twice daily for 5 days
- E. Take it with an antibiotic so it works against the virus
Reveal answer & rationale
Common questions
What causes influenza & viral respiratory illness?
What are the early signs of influenza & viral respiratory illness?
How do I tell a cold from the flu?
Are antivirals useless after 48 hours?
Can someone with an egg allergy get the flu vaccine?
Which flu vaccine recommendations apply to the 2026–27 season?
Sources
- About Influenza — Centers for Disease Control and Prevention
- Signs and Symptoms of Flu — Centers for Disease Control and Prevention
- People at Increased Risk for Flu Complications — Centers for Disease Control and Prevention
- Influenza — StatPearls — StatPearls (NCBI Bookshelf)
- Treating Flu with Antiviral Drugs — Centers for Disease Control and Prevention
- Influenza Antiviral Medications: Summary for Clinicians — Centers for Disease Control and Prevention
- Infection Control in Healthcare Settings — Influenza — Centers for Disease Control and Prevention
- Key Facts About Seasonal Flu Vaccine — Centers for Disease Control and Prevention
- Interim Clinical Considerations for the Use of Seasonal Influenza Vaccines in the United States (2026–2027) — Centers for Disease Control and Prevention
- Clinical Signs and Symptoms of Influenza — Centers for Disease Control and Prevention
Reviewed by Hae Suk Lee, RN
Educational summary for nursing students. Scenarios are fictional teaching cases. Always verify against current guidelines and your institution's protocols before acting in practice. Not medical advice.