Influenza is a virus – a pack of protein and DNA that lacks the capacity to self-reproduce. So it infects a cell, hijacks the inner machinery and uses it to reproduce. The virus reproduces until there are so many copies that the cell bursts and the virus spills out, spreading to other healthy cells.
There are three types of influenza viruses: A, B, and C. Swine flu (H1N1) and the much hyped avian flu (H5N1) are both Type A.
Type A: Infects people, pigs, birds, horses, seals, whales, and other animals. Wild birds are natural hosts. Divided into subtypes based on two surface proteins – hemagglutinin (HA) and neuraminidase (NA). There are 15 HA and 9 NA subtypes, and these can be combined in various ways. Currently, the three most common subtypes in general human circulation are H1N1, H1N2, and H3N2. These can cause epidemics – defined as a high incidence of disease in an area or population – and also a widespread geographic or global disease called a pandemic.
Type B: Normally occurs only in humans. No subtypes. Known to cause human epidemics, but not pandemics.
Type C: Only causes mild respiratory illness in humans, and is not included in flu vaccines. Not capable of epidemic or pandemic spread.
Types A and B are further characterized into genetic variants called “strains.” New strains are constantly evolving and take the place of older ones. While your body may have built up resistance against one strain, it may not be able to fend off its replacement.
