Facts about Influenza
Definitions
Influenza or "flu" is an infection of the respiratory tract that can affect millions of people every year. It is highly contagious and occurs mainly in the late fall, winter, or early spring. Influenza is spread from person-to-person through mists or sprays of infectious respiratory secretions caused by coughing and sneezing. Influenza affects all age groups and causes moderate to severe illness, loss of school and work, and complications such as pneumonia, hospitalization, and death.
Statistics
Each year about 20,000 Americans die because of influenza or influenza related pneumonia. Over 90% of the deaths occur in persons aged 65 years and older.
Pneumonia and influenza together are the 6th most common cause of death in the United States.
Influenza epidemics may be severe and lead to considerable illness and death. A global epidemic, or pandemic, of influenza A in 1918 caused over 20 million deaths worldwide and 500,000 deaths in the United States. Other pandemics occurred in 1957 and 1968. The 1968 Asian influenza pandemic caused over 50,000 deaths in the United States and cost over 3.5 billion dollars in medical costs and lost work days.
Surveillance
There are three surveillance systems, coordinated and maintained by the Centers for Disease Control and Prevention (CDC), to help scientists in the United States evaluate the extent and severity of influenza throughout the year. In one system, the percentage of pneumonia and influenza deaths in 122 cities is compared to a mathematical model of baseline influenza activity. From this data, an "epidemic threshold" can be calculated. When the actual percentage of pneumonia and influenza deaths exceeds the epidemic threshold, public health officials know that a national epidemic of influenza is occurring.
Weekly Pneumonia and Influenza Mortality Chart
Another system relies on weekly reports of influenza-like activity from state health departments. State health departments report widespread, regional, sporadic or no cases of influenza to the CDC.
Epidemiologists' Reports of Influenza Activity
Finally, a network of about 260 physicians across the United States report the weekly number of cases of influenza-like illnesses they see in their practices.
Physicians' Reports of Influenza-like Illness.
The information from these surveillance systems is published periodically by the CDC, and is available on the Internet, in the Morbidity and Mortality Weekly Report and the CDC Influenza Branch Weekly Update.
