What: The influenza A (H7N9) virus is a strain of influenza viruses that normally infect birds. Both the source of infection and the mode of transmission are uncertain.
Symptoms: Most patients with this infection have severe pneumonia. Symptoms include fever, cough and shortness of breath.
How many: As of Tuesday, there have been 28 cases of the H7N9 virus, including nine deaths.
What: H1N1, commonly known as swine flu, is an influenza virus that spreads from human to human. It originated from animals and its most well-known strains are endemic in pigs. It was declared pandemic by the World Health Organisation (WHO) from June 10, 2009, to Aug 10, 2010.
Symptoms: Symptoms include high fever, cough, muscle pains, sore throat, fatigue, vomiting or diarrhoea.
How many: As of May 30, 2010, there were reported laboratory confirmed cases of pandemic influenza in 214 countries and territories of the swine flu strain of H1N1, including more than 18,138 deaths.
What: H5N1 is a type of influenza virus that infects birds. It is commonly called avian influenza (or bird flu). Human cases occur occasionally, human-to-human transmission is difficult. When infected, the human mortality rate is about 60 per cent.
Symptoms: Initial symptoms include high fever, diarrhoea, vomiting, chest pain and bleeding from the nose and gums. Difficulty in breathing develops around five days following the first symptoms.
How many: As of March 12 this year, there have been 622 confirmed cases and 371 deaths since 2003 .
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