There are many different flu viruses that cause different types of infections in human beings, animals and birds. Scientists classify flu viruses into three main categories:
Influenza Type A
This virus acts as a basic host to a number of wild bird species. But in some cases, this virus can also infect mammals, including humans, horses, seals, whales and swine. This virus is posing a major threat to mankind at present, and there is no preventive vaccine available to counter this virus.
Influenza Type B
This virus is primarily known to infect humans. It is highly infectious in nature, and can cause an epidemic if left unchecked. Vaccines are produced every year to counter the infection caused by this type of virus.
Influenza Type C
This class of virus is the most common and benign in nature. It infects human beings, but does not pose a serious threat to life. It can lead to minor and temporary sickness that goes away within a few days on its own.