According to the Centers for Disease Control and Prevention, seasonal (or common) flu is a respiratory illness that can be transmitted person to person. Most people have some immunity, and a vaccine is available.

Avian (or bird) flu is caused by influenza viruses that occur naturally among wild birds. The H5N1 variation is deadly to domestic fowl and can be transmitted from birds to humans. There is no human immunity and no vaccine is available.

H1N1 flu is caused by the H1N1 type A influenza virus that regularly causes outbreaks of influenza in pigs. The H1N1 virus is now spreading person-to-person worldwide. Antiviral drugs may be used for people at high risk for flu.  A vaccination is currently under development.

Pandemic flu is powerful human flu that causes a global outbreak, or pandemic, of serious illness. Because there is little natural immunity, the disease can spread easily from person to person. In 2009, the World Health Organization declared a worldwide pandemic of the H1N1 virus.