Virus

Orthomyxoviruses cause influenza ("flu"). This highly contagious viral infection can quickly assume A transmission electron micrograph (TEM) of Human Herpes Virus type 6 (HHV6) infecting a human cell. This cell was isolated from an AIDS patient suffering a secondary infection with the virus. HHV6 is now known to be the cause of the childhood disease roseola infantum which produces sudden fever, irritability, and a skin rash. Photograph by A. B. Dowsett. Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. epidemic proportions, given the right environmental conditions. An influenza outbreak is considered an epidemic when more than 10% of the population is infected. In 1918, the influenza virus infected 25 million Americans and killed 22 million people worldwide in 18 months. Most people who require hospitalization due to influenza or who die from the infection are elderly individuals, especially those with a pre-existing chronic lung or heart condition. The most common complication of influenza is pneumonia.

Influenza viruses are spherical, single-stranded RNA viruses, with visible protein spikes that protrude from the capsid. Three strains of influenza virus—strains A, B, and C—cause illness in humans. Strain C causes a relatively mild illness that usually does not balloon into an epidemic. Strains A and B cause more debilitating illnesses and are the frequent cause of epidemics. These strains undergo frequent genetic mutation, so that antibodies that are made in the body against prior strains are ineffective against mutated strains. Strain A is the most common influenza virus; strain B emerges only once every two to four years.

A vaccine against the current strain of influenza is prepared each year. This vaccination should be performed during the fall months, so that antibodies against the virus reach optimum numbers by the winter flu season. For reasons that are not clear, flu vaccinations are only 50-60% effective in children, while in the elderly, they can be 70-90% effective. Therefore, most experts recommend that the elderly, who are most at risk for developing serious complications from flu, receive a flu vaccination every year.