Flu viruses are spread by people who have the illness. The most common forms of transmission are coughing and sneezing. Authorities at the Centers for Disease Control and Prevention say that a person with the flu is most infectious beginning the day before the onset of fever and ending 24 hours after the fever subsides. Children and people with immune system deficiencies may be contagious a few days longer.
Spread by Contact
Flu viruses rely upon human contact to spread, and that's why public health officials encourage people to practice good hygiene and to stay home from work or school when stricken by the flu. The viruses hitch a ride on small droplets of saliva that are coughed or sneezed into the air. Breathing in one of these droplets does not ensure that a person will become infected, but it greatly raises the odds. If illness develops, it will happen on an average of two days after exposure to the virus, notes the CDC. It helps to wash hands frequently and cover coughs and sneezes, particularly during flu season. Flu drugs can also be taken to prevent infection, but this precaution is usually limited to health care workers.
Contagious Before Symptoms
Unfortunately, people with the flu become contagious about a day before symptoms really kick in, usually with the sudden onset of a high fever. This so-called "viral shedding" can persist for five to seven days, according to the CDC. In young children and people with weakened immune systems, the period of contagion is slightly longer, notes the British National Health Service. The amount of virus shed reaches its peak in the first two or three days of illness and appears to correlate with a patient's fever, the CDC explains.
Treatment May Lessen Infectiousness
There is some evidence that treatment with antiviral drugs could shorten the length of time that a person is infectious. However, even if people take these drugs, they can spread flu viruses for a few days and perhaps longer. So the CDC recommends that precautions against contagion should be maintained even if a person is taking flu drugs. People at greatest risk of infection are those who remain in close contact with someone who has the flu. The CDC defines "close contact" as sharing eating and drinking utensils or living closely together, like family members. Walking with someone with the flu or sitting in a waiting room across from someone with the flu is not considered "close contact,'' according to the CDC.
