image by Michael Hashizume (lic)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.
Novel H1N1 flu is a new influenza virus causing illness in people. This new virus was first detected around 2009, and has spread to many countries around the world.
Bird flu is commonly used to refer to Avian flu (see below). Bird flu viruses infect birds, including chickens, other poultry and wild birds such as ducks.
Avian flu (AI) is caused by influenza viruses that occur naturally among wild birds. Low pathogenic AI is common in birds and causes few problems. Highly pathogenic H5N1 is deadly to domestic fowl, can be transmitted from birds to humans, and is deadly to humans. There is virtually no human immunity and human vaccine availability is very limited.
Pandemic flu is virulent 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.