When is a child most infectious with influenza?

• A. Twenty-four hours before and after the onset of symptoms.
• B. Twenty-four hours after the onset of symptoms.
• C. One week after the onset of symptoms.
• D. One week before the onset of symptoms.

Answer: (A)

Influenza spreads mainly through droplets from an infected person, and contagiousness begins before symptoms appear and continues after symptoms begin. The amount of virus shed is highest around the time symptoms start, and in children the contagious period can extend longer, but the window around symptom onset remains the most important for transmission.

So, roughly 24 hours before symptoms and the first 24 hours after symptoms begin capture the period when a child is most infectious. This reflects pre-symptomatic spread plus the early symptomatic phase when viral shedding is still high.

Choosing only after symptoms start misses the important pre-symptomatic transmission, and waiting a week after onset overextends into a time when infectivity is typically waning. Being infectious a week before onset ignores the post-onset period entirely.