A virus particle -- or virion -- is a microscopic packet that contains genetic material wrapped in a layer of protein. Some viruses also have a lipid membrane around the protein coat. Unlike bacteria, they cannot reproduce on their own -- they have to invade host cells. This process destroys cells and makes people sick.
Viruses usually enter the bodies of animals and people through their mouths, mucus membranes or breaks in the skin. Then, they infect specific cells. For example, common cold viruses attack cells in the respiratory system. As they reproduce, they destroy their host cells, releasing copies of the virus to attack other cells. Some viruses are more stable than others, but in general they mutate frequently, sometimes making it difficult for doctors to treat them.
There are three types of influenza virus -- types A, B and C. Multiple subtypes exist within those types, and multiple strains exist within each subtype. Like many viruses, influenza can mutate through antigenic drift (small changes that occur as it reproduces) or antigenic shift (major changes that create a new subtype of the virus).
The influenza virus has eight gene segments. When two different types of influenza encounter each other, they can swap segments of their DNA. This can lead to new strains of the virus, some of which can be especially deadly. In fact, scientists believe that the two most recent flu pandemics occurred after human strains of influenza acquired genes from an avian flu virus.
Next, we'll look the strains of influenza that birds carry.