Table of Contents
- Symptoms of Severe Reactions to Vaccinations
- Diphtheria, Tetanus, and Pertussis
- Measles, Mumps, and Rubella
- Varicella-Zoster Virus (Chickenpox)
- Varicella-Zoster Virus (Shingles)
- Hepatitis A
- Hepatitis B
- Pneumococcal Pneumonia
- Viral Influenza
- Haemophilus Influenzae Type B
- Human Papillomavirus (HPV)
- Other Vaccinations
- Vaccinations for Travelers to Developing Countries
Influenza, commonly called the flu, is always caused by a virus.
There are different strains of influenza:
- Influenza A is the most widespread and most severe strain. It can affect both animals and humans. Influenza A is the cause of all the worldwide epidemics (pandemics) of the flu that have occurred. Influenza is responsible for over 200,000 hospitalizations a year in the U.S.
- 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.
The 2009 H1N1 "Swine flu" pandemic was a new strain of influenza A.
Complications of the Flu. Pneumonia is the major serious complication of the flu and can be very serious. It can develop about 5 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, pregnant women, and hospitalized or immunocompromised patients.
The largest number of H1N1 flu cases occurred in people ages 5 - 24. Fewer cases and deaths have been reported in people older than age 64. This is a different pattern than that seen with the seasonal or regular flu outbreaks.
[For more information, see In-Depth Report #94: Colds and the flu.]
Review Date: 10/12/2010
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.