Flu is a contagious respiratory tract infection caused by Influenza viruses. Influenza viruses are of three types A, B and C. Type A has caused most human outbreaks whereas type B and C are less often associated with severe human infections.

Swine flu is a respiratory infection seen in pigs due to Influenza viruses. They spread among pigs due to mutual contact but usually do not spread to the Human beings. It may spread to humans due to close contact with the pigs or environments contaminated with swine influenza viruses. Subsequently human to human spread may occur via coughing and sneezing.

Influenza viruses have two surface antigens (proteins) - the H (hemagglutinin) and the N (neuraminidase). These help the virus to adhere to the respiratory tract lining. These H and N antigens also have some difference structurally between virus groups to virus group. These differences are conveyed by a specific number assigned to each antigen. Various combinations of differently numbered antigens give strain specificity to the viruses. eg. The SARS outbreak was caused by the strain of influenza virus A H5N1. The A H1N1 has been usually infecting the pigs only and this is the first major outbreak of this new virus. Hence humans do not have immunity against this virus strain.

Cases of influenza-like illness were first reported in Mexico on March 18; the outbreak was subsequently confirmed as swine influenza A. As of 1600 GMT, 3 May 2009, 18 countries have officially reported 898 cases of influenza A (H1N1) infection. Mexico has reported 506 confirmed human cases of infection, including 19 deaths. The United States Government has reported 226 laboratory confirmed human cases from 30 states, including one death. The following countries have reported laboratory confirmed cases with no deaths - Austria (1), Canada (85), China, Hong Kong Special Administrative Region (1), Costa Rica (1), Denmark (1), France (2), Germany (8), Ireland (1), Israel (3), Italy (1), Netherlands (1), New Zealand (4), Republic of Korea (1), Spain (40), Switzerland (1) and the United Kingdom (15). So far no cases have been reported from India.

An influenza pandemic occurs when a new influenza virus appears against which the human population has no immunity, resulting in epidemics worldwide with enormous numbers of deaths and illness. With the increase in global transport, as well as urbanization and overcrowded conditions, epidemics due the new influenza virus are likely to quickly take hold around the world. Currently the WHO has declared a phase 5 pandemic level for influenza H1N1. (Phase 5 - Still not a pandemic, but spread of disease between humans is occurring in more than one country of one WHO region).