Respiratory Virus Symptoms: Cold, Flu or RSV?

Respiratory viruses share a symptom list — runny nose, sore throat, cough, fever — and separate mostly by how they start and how hard they hit. A cold builds gradually and stays in the head; the flu arrives suddenly with fever and body aches; RSV is a mild cold in an adult and a potentially serious illness in an infant.

Four illnesses, one complaint

• Common cold. More than 200 different viruses can cause one, MedlinePlus notes, with rhinoviruses the most frequent. Sneezing, congestion, runny nose, sore throat, cough and headache, beginning a few days after infection and lasting up to 10 to 14 days.
• Influenza. Symptoms come on suddenly: fever, cough, sore throat, runny nose, muscle aches, headache and fatigue.
• RSV. Usually mild and cold-like in older children and adults — runny nose, cough, sneezing, fever, wheezing. Most infections clear in one to two weeks, and RSV season in the U.S. runs fall through spring.
• COVID-19. Overlaps with all of the above and can also present without respiratory symptoms at all.

What separates a cold from the flu

	Cold	Flu
Onset	Gradual	Sudden
Fever	Rare	Usual
Body aches	Slight	Typical
Dominant complaint	Nose and throat	Whole body
Specific treatment	None	Antivirals, best within 2 days

MedlinePlus draws exactly these distinctions. They’re useful but not diagnostic — a bad cold can feel like a mild flu, and the reason to care is that influenza has a treatment window a cold doesn’t.

The symptoms almost all of them produce

• Sore throat, often the first thing you notice — see sore throat causes
• Nasal congestion and discharge
• Cough, dry at first and frequently the last symptom standing; see persistent cough causes
• Headache and fatigue
• Fever, common with flu, uncommon with a cold

In babies, RSV changes what you watch for

This is the piece adults get wrong. MedlinePlus notes that very young infants with RSV may show only irritability, reduced activity and difficulty breathing — not the runny nose and cough you’d expect. Babies born prematurely or with congenital heart disease, chronic lung disease or a weakened immune system are at highest risk, since RSV can progress to bronchiolitis or pneumonia.

Any sign of effort to breathe — fast breathing, flaring nostrils, the skin pulling in between the ribs, pauses in breathing — is a same-day evaluation in an infant, whatever the thermometer says. A baby under 3 months with a temperature of 100.4°F (38.0°C) or higher needs a call regardless.

Treatment: what exists and what doesn’t

For most respiratory viruses, MedlinePlus says treatment can only help with symptoms while your immune system fights the infection off. Influenza is the partial exception: antiviral medicines can make the illness milder and shorten it, and they work best started within two days of symptom onset — which is why a sudden high fever is worth a same-day call rather than a wait-and-see week. An annual flu vaccine remains the main prevention, and it covers influenza only, not the other viruses on this page.

When to seek care

• Trouble breathing, fast breathing, or chest pain
• A fever that won’t come down with medication, or one lasting more than three days
• Symptoms that improve and then clearly worsen — a possible secondary bacterial infection
• Confusion, unusual drowsiness, or dizziness on standing
• Any breathing difficulty in an infant, an older adult, or someone with asthma or COPD

For the seasonal picture and the viruses that peak when these don’t, see summer virus symptoms, and for the wider frame, viral infection symptoms.

This content is informational and does not replace medical evaluation, diagnosis, or treatment.