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. A seasonal influenza outbreak generally occurs during the winter months of the year and frequently happens in places such as schools, nursing homes and other areas where people come into close contact. The infectious influenza Type A and B viruses responsible for most seasonal flu outbreaks can develop into community and regional epidemics. These annual outbreaks are caused by strains of the viruses that people have some natural immunity to, resulting in epidemics that do not usually cause high mortality rates. This differs from pandemics, which are characterized by the spread of new strains of flu virus that people have no immunity to.
A pandemic influenza outbreak begins with a new strain of influenza Type A virus. Sources of new strains include domestic and wild birds, pigs and other animal species. Problems start when the animal flu virus is transmitted to a human. For an influenza outbreak to occur, this new strain must be capable of passing from human to human. People have built up no 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 influenza 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 made ill 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 globally.
Frequent global travel is a factor influencing the spread of an influenza outbreak. Business, tourist and refugee travel, along with international shipments of animals, help pass the influenza virus more rapidly than in the past. The virus is often spread via travel before it has even been detected. Once a new strain of influenza virus has been identified, shipments of carrier animals are monitored or ceased. Natural bird migration patterns may also affect the spread of viruses having the potential to be transmitted to humans.
Although symptoms of the seasonal flu are generally mild, some segments of the population are more at risk than others. The elderly and those with chronic health conditions may have more severe symptoms. Aches and pains, chills and a fever are the most common symptoms, but those at risk may develop life-threatening complications. Simple strategies to stop the spread of the virus include frequent hand washing, avoiding contact with infected individuals, and covering a sneeze or cough with a tissue. Each season, influenza vaccinations are available that boost immunity to the strains that are expected to circulate.
