Influenza Viruses (Flu) are infectious diseases that strike birds and mammals and are largely composed of three types: A, B, and C. These viruses spread through people from respiratory droplets of coughs and sneezes, either through the air by passing into the nose or mouth, or by touching an object containing droplets and then transferring it by touch to the eyes, nose, or mouth. The symptoms include mild to severe illness, and sometimes death. (Protection tips)
A recent influenza virus causing illness in people is the H1N1 (Swine) Flu. The H1N1 virus is now considered a pandemic by the World Health Organization. The range of symptoms vary from mild to severe.
Avian Influenza (Bird Flu) is very contagious in birds and can be transmitted to humans. H5N1 is a subtype that has crossed into humans and can cause death; however, the efficient transmission from humans to humans has not occurred. Should this happen, we are likely to experience a pandemic.
CDC links on other infectious diseases