Reviewed by Dr. Clement J. Cheng
Influenza, more commonly known as the flu, is a contagious viral infection of the respiratory tract that causes weakness, fever, headaches, and muscle aches. The flu is often confused with the common cold, which is a viral infection caused by one of over 200 rhinoviruses—a completely different virus than influenza.
Influenza is a serious condition that can lead to secondary infection, bronchitis and pneumonia. On average, 20,000 people in the United States die each year from causes related to influenza; those at the highest risk are the elderly and immuno-comprimised.
Three types of influenza virus exist. Type A is a constantly changing virus, responsible for large outbreaks as new strains develop. Type A is the virus type that causes a flu epidemic every few years. Type B and C are more stable viruses—Type B causes a smaller outbreak than Type A, and Type C causes symptoms more analogous to a common cold.