What factors can cause an outbreak of influenza?

Factors that cause an influenza outbreak include seasonal weather patterns, levels of immunity to the specific virus strain, and the incidence of human contact that spreads the virus. An outbreak of seasonal influenza usually occurs during the winter months of the year and often occurs in places like schools, nursing homes, and other areas where people come into close contact. Infectious influenza A and B viruses responsible for most seasonal flu outbreaks can become community and regional epidemics. These annual outbreaks are caused by virus strains to which people have some natural immunity, resulting in epidemics that generally do not cause high mortality rates. This differs from pandemics, which are characterized by the spread of new strains of the flu virus to which people have no immunity.

An outbreak of pandemic influenza begins with a new strain of type A influenza virus. Sources of new strains include domestic and wild birds, pigs, and other animal species. The problems begin when the animal flu virus is transmitted to a human. For an influenza outbreak to occur, this new strain must be able to pass from human to human. People have not developed natural immunity to this new strain of virus, so everyone is susceptible to the disease, causing it to spread rapidly.

Records show that pandemics are relatively rare, with only a few occurring each century. During the 20th century, the most famous flu pandemic swept the world from 1918 to 1919. It is estimated that up to 40 million people may have died during this pandemic, while hundreds of millions were sickened by the virus. Other pandemics during the 20th century include the Asian flu pandemic of the late 1950s and the Hong Kong flu pandemic of the late 1960s. The swine flu outbreak of 2009 and 2010 also reached pandemic levels when a new strain of influenza type A spread worldwide.

Frequent global travel is a factor in the spread of an influenza outbreak. Business, tourist, and refugee travel, along with international animal shipments, help transmit the influenza virus more rapidly than in the past. The virus is often transmitted through travel even before it has been detected. Once a new strain of influenza virus has been identified, shipments of carrier animals are monitored or ceased. Natural bird migration patterns can also affect the spread of viruses that have the potential to be transmitted to humans.

Although seasonal flu symptoms are generally mild, some segments of the population are at higher risk than others. The elderly and people with chronic health conditions may have more severe symptoms. Aches and pains, chills, and fever are the most common symptoms, but those at risk can develop life-threatening complications. Simple strategies to stop the spread of the virus include frequent hand washing, avoiding contact with infected people, and covering a sneeze or cough with a tissue. Each season, flu vaccines are available that boost immunity to strains expected to circulate.

Go up