The flu, or seasonal influenza, is a highly contagious infection caused by the influenza virus. The flu virus typically infects the respiratory tract, including the nose, throat and lungs. Since the flu is an airborne virus, it often affects many people at once. The Centers for Disease Control and Prevention, or CDC, estimates that 5 to 20 percent of people in the United States will contract the flu in a given year. MayoClinic.com reports the most effective prevention against flu is a vaccine administered annually.
Influenza Virus
The flu is caused by a viral infection. The illness is contracted after coming into contact with the live strain of the influenza virus. According to MayoClinic.com, the flu virus is highly mobile and is able to travel in droplets in the air from an infected person coughing, sneezing or talking. The flu is also spread by coming in contact with a doorknob, telephone or keyboard -- any surface with the virus on it -- and then touching your own eyes, mouth or nose.
Flu Strains and Seasonal Epidemics
MayoClinic.com reports that flu viruses are constantly changing and developing into new strains. While antibodies, developed as a result of past flu episodes, may protect from similar strains, the constant evolution of the virus makes it nearly impossible to develop immunity.
According to the World Health Organization, flu outbreaks are often characterized by seasonal influenza epidemics, occurring mostly during late fall, winter or early spring. Some strains of the flu, such as influenza B, may appear anytime throughout the year.
High-risk Groups
While the flu may infect anyone, certain groups are at a higher risk of contracting the illness; these groups include young children, the elderly, pregnant women and people with weakened immune systems or other medical conditions. The CDC also reports that these high-risk groups are most likely to suffer flu-related complications, including pneumonia and bronchitis.
