Colds and the Flu - Introduction

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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.

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.

Based on a final analysis of the 2001 - 2002 flu season, nearly 90% of flu cases were type A, and about 10% were type B. Influenza A usually causes more severe disease than type B. There is some concern, however, that since influenza B has been less common in the past few years, some people, particularly small children, may have fewer antibodies to it and so may be at higher risk for severe infection.

Avian Influenza (Bird Flu)

Although the risk of lethal viruses is generally low, scientists are greatly concerned about a particular virus called H5N1, which causes avian influenza (bird flu). Since 1997, the H5N1 virus has triggered deadly outbreaks in poultry across Southeast Asia. As of January 22, 2007, 269 people had been infected with the bird flu in 11 countries. More than half of these people have died, according to the World Health Organization. No cases have been seen in the United States.

So far, the virus has spread from birds to humans. The virus does not seem to be easily spread from person to person. However, scientists and public health officials are monitoring the spread of H5N1 and working to contain it. Efforts include slaughtering infected birds, developing new vaccines, and stockpiling antiviral drugs such as oseltamivir (Tamiflu). Many poor nations have limited resources and already contend with other serious health problems, including HIV-AIDS. If H5N1 does mutate and spread, the consequences could be especially severe for these countries.

Review Date: 03/06/2007
Reviewed By: A.D.A.M. Editorial Team: Greg Juhn, M.T.P.W., David R. Eltz, Kelli A. Stacy. Previously reviewed by Harvey Simon, MD, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital (12/4/2006).