Community Health - Understanding Influenza

Influenza is an upper respiratory (breathing) disease that is caused by viruses. Flu is different than the common cold in several ways. Flu usually causes a fever, headache and exhaustion, which are very rarely caused by the common cold. Influenza is also confused with gastroenteritis (the “stomach flu”). While nausea and vomiting are possible with flu, especially in children, they are less common in adults and will always accompany fever and respiratory symptoms.

The CDC estimates that 5-20% of Americans come down with the flu each flu season. Flu season typically lasts from November to March in the U.S. Children are 2-3 times more likely to get sick with the flu, and often spread it to other children. For these reasons, families with school aged children or children in day care typically have more infections of the flu than other families. Although most people recover from the flu without complications, the CDC estimates that more than 200,000 U.S. residents are hospitalized and about 36,000 people die from the flu and its complications each year.

SEASONAL FLU Seasonal flu is a term used to refer to the different types of flu virus that circulate on a yearly basis. Flu season is the late fall, winter and early spring. In Ohio, flu season generally peaks in January and February.

PANDEMIC FLU Pandemic flu refers to particularly infectious strains (types) of flu that spread easily from person to person to create a worldwide epidemic or pandemic. Pandemic flu spreads so efficiently because we do not have immunity to the new flu strain. We are currently in a pandemic. See also How does pandemic flu differ from seasonal flu?

AVIAN (BIRD) FLU Influenza naturally occurs in aquatic birds and shore birds. Normally it does not spread to humans. However, pigs can be infected by bird influenza and by human influenza. The bird and human influenza viruses can combine to form a new type of flu. The pig can then pass the resulting avian flu to a human. In 1997, it was discovered that a strain of bird flu had transmitted to humans directly, skipping the pig step. This caused a number of deaths in Asia, but the virus was unable to pass effectively from person to person. This type of flu causes concern because if at some point a strain was able to pass human to human, it could cause a pandemic.

Influenza is one of the most changeable viruses. Changes in flu viruses can be small and continuous (drift) or large and abrupt (shift). The first is called Antigenic Drift; these changes happen when the virus makes copies of itself. Because viruses change over time, a new flu vaccine is needed each year. Drift happens constantly.

The second way flu changes is called Antigenic Shift. The virus also changes suddenly when two different strains of flu infect a person or animal. The two viruses then combine genes from each other to form a new type of flu. Because people have little to no immunity to this new type, it often causes an epidemic or pandemic. This change does not happen often.