Medical Health Encyclopedia

Symptoms.  Patients usually feel sick 1 to 4 days after exposure to the influenza (flu) virus. The flu usually involves:

• Abrupt onset of severe symptoms, which include headache, muscle aches, fatigue, and high fever (up to 104 degrees F).
• Cough (which is usually dry but can be severe) and sometimes a runny nose and sore throat.
• Children may experience vomiting, diarrhea, and ear infections, as well as other flu symptoms.
• The symptoms usually resolve in 4 to 5 days, although some people can experience coughing and feelings of illness for more than 2 weeks. In some cases, flu can become more severe or make other conditions worse. 

Transmitting the Virus. The virus is spread primarily when a person with the flu coughs or sneezes near someone else. Adults with flu typically spread it to someone else from 1 day before symptoms start to about 5 days after symptoms develop. Children can spread the infection for more than 10 days after symptoms begin, and young children can transmit 6 days or even earlier before the onset of symptoms. People with severely compromised immune systems can transmit the virus for weeks to months.

Flu Strains. A virus is a cluster of genes wrapped in a protein membrane, which is coated with a fatty substance that contains molecules called glycoproteins. Strains of the flu are identified according to the number of membranes and type of glycoproteins present.

Click the icon to see an image of a virus.

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The two major flu strains are referred to as A and B:

• Influenza A is the most widespread and can even infect animals and humans. Influenza A is the cause of the major pandemics (worldwide epidemics) of influenza that have occurred. It is usually further categorized by two subtypes based on two substances that occur on the surface of the viruses: hemagglutinin (H) and neuraminidase (N).
• Influenza B infects only humans. It is less common than Type A, but is often associated with specific outbreaks, such as in nursing homes.