There are three types of influenza or flu, namely A, B, C. Among the three types, type A is the most dangerous. There is a strong belief that Type A has been responsible for many global outbreaks like in 1918, 1957, 1968, etc. In addition to these types, the viruses have been classified based on the place and year of discovery of the strain, type of HA, NA proteins, lab identification number, etc.

The first influenza virus was discovered in 1930s. Scientists have categorized those viruses from that time into three main types, as already mentioned, depending on the composition of proteins they possess.

Type A virus have been discovered in many animals like pigs, ducks, chickens, whales and humans. Type B is found mostly in humans alone and type C is found in pigs, dogs and humans. Type C is responsible only for mild infections unlike types A and B. Type C do not lead to epidemics.
Type A viruses can still be subdivided into subgroups depending on the two surface proteins, namely HA and NA. Research has subdivided into 16 HA types and 9 NA types of the influenza type A virus. The subtypes are based on a naming system including the place and year of discovery of the strain, type of HA, NA proteins and lab identification number.

An apt example is A/Hong Kong/156/97 (H5N1), where A represents the type A virus, Hong Kong is the place of discovery of the subtype, 156 is the lab identification number, 97 is the time of discovery, H5N1 represents the virus has 5 HA proteins and 1 NA protein. If the flu virus infects animals other than humans, it is represented by A/Chicken/Hong Kong/G9/97 (H9N2). Type B and C have no subdivisions.

The flu viruses are naturally found in wild ducks, shore birds, etc. Though the virus has been in such birds for several millions of years, it does not harm them. The flu virus can easily jump the species barrier and infect domesticated animals in an order like ducks, chickens, pigs, etc. Pigs are affected by both avian flu and human flu. Hence in a farm where these animals and birds live in close proximity, pigs do represent flu virus-mixing bowl. When a pig is affected by both the human and avian flu at the same time, the viruses can exchange genes. Such a virus can infect human from pigs.
Based on the range of bird-type flu surface proteins that get exchanged into the humans, the influenza disease may vary from mild to severe. In 1997, this step has been surpassed and the bird flu can affect humans directly without the pigs acting as bowls. Such an outbreak in 1997 was assumed to turn into a pandemic but luckily the virus was unable to pass between humans.