RSV vs. the common cold vs. flu: how to tell them apart

In infants, the early symptoms of RSV, common cold, and influenza overlap almost completely. Runny nose, cough, mild fever, fussiness — they all start this way. But they follow different paths and carry different risks, which is why understanding the distinctions matters — especially for parents of newborns and young infants.

The common cold

Caused by any of 200+ viruses (most commonly rhinoviruses). In healthy infants, colds are generally mild:

• Gradual onset over 1–2 days
• Runny nose, nasal congestion, mild cough
• Low-grade fever or no fever
• Typically resolves in 7–10 days
• Rarely causes lower respiratory illness in otherwise healthy infants

The cold is the baseline comparison. If your baby has "just a cold," the lower airways are not involved and breathing should remain comfortable.

RSV

RSV starts like a cold but has a higher probability of moving into the lungs in young infants:

• Starts with upper respiratory symptoms — runny nose, cough, mild fever
• Typically peaks around days 3–5
• In young infants, may progress to bronchiolitis — inflammation of the small airways in the lungs
• Wheezing, fast or labored breathing are RSV's distinguishing features at this stage
• In very young infants (under 2 months): may cause apnea (pauses in breathing) — sometimes before other symptoms develop
• Most children recover in 1–2 weeks; hospitalization needed in ~2–3% of infants under 12 months

Clinically, RSV can only be confirmed with a test (rapid antigen test or PCR nasal swab). Your pediatrician can test for it. However, during RSV season, bronchiolitis in a young infant is treated as RSV until proven otherwise.

Flu (influenza)

Flu in infants tends to have a more abrupt onset and higher fever:

• Sudden onset — often goes from fine to sick quickly
• Higher fever — often 102°F+ in infants
• Muscle aches and body aches (hard to detect in infants — look for increased fussiness, reluctance to move)
• More fatigue/lethargy than a typical cold
• Vomiting and diarrhea are more common with flu than RSV
• Flu can also cause lower respiratory disease in infants, including pneumonia
• Unlike RSV, antivirals (oseltamivir/Tamiflu) are effective against flu if started within 48 hours

Key distinguishing features at a glance

Does it matter which one it is?

For clinical decision-making, sometimes yes:

• If it's flu and you catch it within 48 hours of symptom onset, Tamiflu can shorten the illness and reduce severity — worth testing for
• If it's RSV, there's no antiviral. Management is supportive. But knowing it's RSV helps your pediatrician triage how closely to monitor for lower respiratory progression
• If it's a cold, watchful waiting is usually the right call

During RSV season, any infant with wheezing, fast breathing, or difficulty feeding should be seen by a pediatrician regardless of which virus is causing it — because the management is driven by the clinical picture, not the lab result.

Not medical advice. If your infant is having trouble breathing, contact your pediatrician or call 911.