The influenza virus

The specific type of influenza is determined by the type of influenza virus (A, B or C) that is causing the illness. Types A and B influenza viruses are responsible for most of the world's epidemics (regional outbreaks) and pandemics (worldwide outbreaks) which can cause mass hospitalisation and death. Type B viruses are slightly less severe than type A. Type C infections are even milder, sometimes have no symptoms, and don't cause epidemics. Over time, influenza viruses constantly change by mutation (a change in their viral genetic material). A flu virus is like a little ball covered with pointy spikes. The spikes are proteins called antigens; these attach to cells in a healthy person and alert their immune system to make antibodies to battle the virus.

Every virus strain has its own unique antigens. Your immune system makes antibodies for each kind of antigen it comes into contact with, "memorizing" the antigen so your body can fight off viruses that come back to reinfect you. Unfortunately for us, flu antigens change little by little over time - this is called "antigenic drift." When they change, your immune system doesn't recognize them anymore, and you get sick again if you come into contact with them. This is why people can get the flu continually during their lifetime - they don't become permanently immune once they've had it.

But the real danger happens when antigens suddenly change significantly - this is called "antigenic shift" (it's only happened so far in Type A viruses). When antigenic shifts occur, a pandemic can occur. Pandemics sicken and kill millions of people all over the world because very few people have the right antibodies to fight it off, and flu vaccines are ineffective. Influenza can strike large numbers of people, even entire populations, who have inadequate immune protection especially against a newly mutated virus.