Immunizations - Viral Influenza

Highlights	Hepatitis A	Haemophilus Type B
Introduction	Hepatitis B	Smallpox
Diptheria, Tetanus, and Pertussis	Poliomyelitis	Other Vaccinations
Measles, Mumps, and Rubella	Pneumococcal Pneumonia	Resources
Varicella-Zoster Virus (Chickenpox)	Viral Influenza	References

Viral Influenza

Influenza, commonly called the flu, is always caused by a virus.

Influenza, also known as the flu, is caused by a virus.

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There are different strains of influenza:

Influenza A is the most widespread and most severe strain. Itcanaffect bothanimals and humans. Influenza A is the cause of the worldwide epidemics (pandemics)of the flu that have occurred. An estimated 142,000 hospitalizations per year are due to this strain of the flu. Influenza A 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. Flu caused by this strain tends to be milder than that caused by Influenza A.

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

Complications of the Flu. In general, the flu is usually self-limited and not serious. It is responsible, however, for 15 - 30% of the excess number of hospitalizations that occur in winter. About 1% of people who contract the flu end up in the hospital, and an estimated 36,000 people currently die each year of flu-related complications. The highest risks for serious complications occur in people age 65 and older and in those who are already sick with another disease.There have also been reports of flu-related deaths in very young children.

Pneumonia is the major serious complication of the flu and can be very serious. It can develop about five days after viral influenza. It is an uncommon event, however. It nearly always occurs in high-risk individuals, such as the very young or very old, and hospitalized or immunocompromised patients.

Note on Pandemics. Every year,flu strikes millions of people worldwide. Influenza epidemics are most serious when they involve a new strain against which most people are not immune. Such so-called pandemics can infect more than one fourth of the world''s population within a three-month period. For example, the Spanish flu in 1918 and 1919 killed 20 million people in the US and Europe and 17 million in India. Although pandemics are still of great concern, there have been major improvements in private and public health since then, including the discovery of antibiotics to treat bacterial complications, new anti-viral agents and vaccines, and intensive world-wide surveillance of outbreaks.