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A vaccine is a biological preparation used to stimulate the production of antibodies and provide immunity against one or several diseases. It is prepared from weakened or killed forms of the causative agent of a disease, its products, or a synthetic substitute, treated to act as an antigen without inducing the disease. After treatment with a vaccine, the body's immune system has learnt to recognize the agent as foreign, destroy it, and "remember" it, thus being ready to face future encounters.
Vaccines can be prophylactic (e.g. to prevent or ameliorate the effects of a future infection by any natural or "wild" pathogen), or therapeutic (e.g. vaccines against cancer are also being investigated).
Avian flu, also known as “bird flu”, is an infection caused by avian influenza A viruses. These influenza A viruses occur naturally among birds. Wild birds worldwide get flu A infections in their intestines, but usually do not get sick from flu infections. However, avian influenza is very contagious among birds and some of these viruses can make certain domesticated bird species, including chickens, ducks, and turkeys, very sick and kill them.
Most avian influenza viruses do not cause disease in humans. However, some are zoonotic, meaning that they can infect humans and cause disease. The most well known example is the Highly Pathogenic Influenza A (H5N1) virus – also called "HPAI H5N1 virus" – currently circulating in poultry in parts of Asia and northeast Africa. It is highly contagious among birds and can be deadly to them. Infections with these viruses have occurred in humans, especially in people having direct or close contact with H5N1-infected poultry or H5N1-contaminated surfaces. HPAI H5N1 virus can infect the respiratory tract of humans causing severe respiratory illness (e.g. pneumonia and respiratory failure) and eventually leading to death. Of the few avian influenza viruses that have crossed the species barrier to infect humans, HPAI H5N1 has caused the largest number of detected cases of severe disease and death in humans. More than 600 human HPAI H5N1 cases have been reported to WHO from 15 countries in Asia, Africa, the Pacific, Europe and the Near East since November 2003. Approximately 60% of the cases have died. The majority of these cases have occurred among children and adults younger than 40 years old. Mortality has been highest in people aged 10-19 years old and young adults. Most human HPAI H5N1 cases have presented late in their illness for medical care and have been hospitalized with severe respiratory disease. However, some clinically mild HPAI H5N1 cases have been reported, especially in children.