How does 2009 H1N1 flu spread? Both 2009 H1N1 flu and seasonal influenza are thought to spread mostly from person to person through the coughs and sneezes of people who are sick with influenza. People also may get sick by touching something with flu viruses on it, and then touching their mouth or nose. How long can a sick person spread 2009 H1N1 flu to others? People infected with 2009 H1N1 flu shed virus and may be able to infect others from 1 day before getting sick to about 7 days after getting sick. This can be longer in some people, especially children and people with weakened immune systems. How severe is illness associated with this 2009 H1N1 flu virus? 2009 H1N1 flu illness has ranged from mild to severe. Most healthy people who have been sick with 2009 H1N1 have recovered without needing medical treatment, however, hospitalizations and deaths from 2009 H1N1 have occurred. Most people who have been hospitalized with 2009 H1N1 have had a medical condition that places them at higher risk of serious flu-related complications. However, some people who have become very ill have been previously healthy. Severe infections have been reported among people of all ages.