Croup: AKA laryngitis. Used to be seasonal but we typically see it year round. No time restrictions needed here.
Influenza: Influenza is truly a respiratory illness and not a gastrointestional (GI) one. You know an author has thoroughly checked their facts when they have a summer illness with vomiting and diarrhea and don't call it influenza. This is why flu shots are given Sept-Nov... to help prevent the transmission of this illness.
Keep seasonal illnesses in mind when you're writing a novel that occurs during a specific time of year. It may behoove you to ask if that contagious illness you're killing off your characters with would actually occur during your time frame.
Have you used a contagion, known or "created" in your novel?