Seasonal flu sickens millions of Americans each year. The Centers for Disease Control and Prevention (CDC) reports that the incubation period--the interval between exposure to the virus and the appearance of symptoms--averages two days, with a range of one to four days. However, some symptoms take longer to develop than others. Most people become infectious one day before they become symptomatic.
Systemic Symptoms
According to Dr. Raphael Dolin in "Harrison's Principles of Internal Medicine," systemic symptoms of flu such as fever, headache and muscle pain are among the presenting symptoms of flu. They tend to begin abruptly and at the same time, so much so that many patients can pinpoint the exact time they fell ill. Fever due to flu usually ranges from 100.4 to 105.8 degrees F and peaks about 24 hours after it starts.
Respiratory Symptoms
The dry, hacking cough associated with flu starts at the same time as systemic symptoms. The incubation period for other respiratory symptoms of flu such as sore throat, runny nose and nasal congestion is about one to two days longer, according to the authors of a 2009 report in the Journal of Clinical Microbiology and Infections. Breathing problems due to flu also tend to develop a little bit later--usually after the person has already started to feel better, in the case of flu-related pneumonia.
Gastrointestinal Symptoms
Gastrointestinal symptoms of flu such as nausea, vomiting and diarrhea occur more frequently in children compared to adults, but up to 40 percent of adults experience vomiting, diarrhea or both with the new H1N1 virus, according to a 2009 report in the New England Journal of Medicine. The incubation period for these symptoms is one to three days longer than other kinds of symptoms. They also resolve a little sooner and patients usually rate them as mild.
References
- CDC: Q and A about Seasonal Flu
- "Harrison's Principles of Internal Medicine" 17th edition; A.S. Fauci, et. al.; 2008
- Clinical Microbiology and Infections; "Dynamics of Clinical Symptoms in a Case with Pandemic Influenza A (H1N1)"; CS Lee and JH Lee; Nov. 13 2009
- New England Journal of Medicine; "Emergence of a Novel Swine-Origin Influenza A (H1N1) Virus in Humans"; Novel Swine-Origin Influenza A (H1N1) Virus Investigation Team; June 18 2009
