According to information from the Centers for Disease Control and Prevention (CDC):
"The estimated incubation period is unknown and could range from 1-7 days, and more likely 1-4 days.
"The typical incubation period for influenza is 1-4 days (average: 2 days). Adults shed influenza virus from the day before symptoms begin through 5-10 days after illness onset. However, the amount of virus shed, and presumably infectivity, decreases rapidly by 3-5 days after onset in an experimental human infection model. Young children also might shed virus several days before illness onset, and children can be infectious for 10 or more days after onset of symptoms. Severely immunocompromised persons can shed virus for weeks or months.
"Uncomplicated influenza illness typically resolves after 3-7 days for the majority of persons, although cough and malaise can persist for >2 weeks. However, influenza virus infections can cause primary influenza viral pneumonia; exacerbate underlying medical conditions (e.g., pulmonary or cardiac disease); lead to secondary bacterial pneumonia, sinusitis, or otitis media; or contribute to coinfections with other viral or bacterial pathogens."
Other responses from WikiAnswerers:
- Studies are continuing to determine the specific incubation period of the 2009 swine flu outbreak, but opinions of slight variations in the estimates are surfacing. For now, however, the official CDC information is as quoted above.
- Infected people may be able to pass on the flu to someone else ("shed virus") before they know they are sick, while they are sick, and for varying time periods after feeling well again.
- People with swine influenza virus infection should be considered potentially contagious as long as they are symptomatic and possibly for up to 7 days following illness onset. Children, especially younger children, might potentially be contagious for longer periods.
- Some viruses and bacteria can live 2 hours or longer on surfaces like cafeteria tables, doorknobs, and desks. Frequent hand washing will help you reduce the chance of getting contamination from these common surfaces. Don't go to public places if you have the flu symptoms.
- For treatment, antiviral drugs work best if started soon after getting sick (within 2 days of symptoms).
- This virus has the same properties in terms of spread as seasonal flu viruses. With seasonal flu, studies have shown that people may be contagious from one day before they develop symptoms to up to 7 days after they get sick. Children, especially younger children, might potentially be contagious for longer periods.
Update 9/27/09
A Department of Homeland Security (DHS) report from 9/19/09 indicates that the period of infectivity of A-H1N1/09 is longer than originally believed, and longer than that of the seasonal flu.
Studies on how long a person remains contagious back up the previously reported CDC guidelines that one should be suspected to be capable of spreading the Novel Swine Flu for one full week after the symptoms start or until 24 hours after the fever subsides [while taking no fever reducers] which ever is longer, rather than the most recent CDC guidelines that waiting for 24 hours after fever subsides without taking fever reducers is long enough.
In fact, the best determination can probably be made by how long the cough lasts.
The studies show that in some cases the virus remains in the nasal passages of patients for up to 16 days after symptoms begin. Below are some excerpts from the report:
"Swine flu also appears to be contagious longer than ordinary seasonal flu, several experts said.
When the coughing stops is probably a better sign of when a swine flu patient is no longer contagious, experts said after seeing new research that suggests the virus can still spread many days after a fever goes away.
Using a very sensitive test to detect virus in the nose or throat, [the study] found that 80 percent had it five days after symptoms began, and 40 percent seven days after. Some still harbored virus as long as 16 days later. How soon they started on antiviral medicines such as Tamiflu made a difference in how much virus was found, but not whether virus was present at all. . .
Doctors know that people can spread ordinary seasonal flu for a couple of days before and after symptoms start by studying virus that patients shed in mucus. The first such studies of swine flu are just coming out now, and they imply a longer contagious period for the novel bug. "
1-2 weeks. See related question below for more information.