Feb 1 (Reuters) - The H5N1 avian influenza virus has flared up again in recent months, spreading through poultry flocks in Japan, Vietnam and Thailand, killing people in Indonesia and claiming its first human life in Nigeria.

Here are some key facts about the different types of flu viruses and their behaviour.

HOW MANY TYPES OF INFLUENZA VIRUSES ARE THERE?

- Three: A, B and C.

WHAT'S THE DIFFERENCE?

- Only groups A and B are of concern for humans, as type C infections only cause mild respiratory illnesses.

- Type A, the source of annual flu outbreaks, can cause pandemics. Type B can cause epidemics, but not pandemics.

- Only influenza A viruses and its subtypes can infect birds.

DO THEY ALL HAVE SUBTYPES?

- No. Only influenza A viruses are classified by subtype, according to the two main surface glycoproteins hemagglutinin (HA) and neuraminidase (NA).

WHAT DO THE SUBTYPES SIGNIFY?

- Influenza A viruses have 16 H subtypes, which govern the ability of the virus to bind to and enter the cells of a host, such as humans, to multiply.

- They also have nine N subtypes, which govern the release of newly replicated virus from the host's cells.

WHICH ARE THE DANGEROUS SUBTYPES?

- Only viruses of the H5 and H7 subtypes are known to cause the highly pathogenic form of bird flu. And not all of these will cause severe disease in poultry.

- Subtypes of influenza A currently circulating worldwide include H1N1, H1N2, and H3N2 viruses.

HOW DO VIRUSES CHANGE?

- There are two different ways: antigenic drift, which happens all the time, and antigenic shift, which only happens occasionally.

- Type A flu viruses undergo both kinds of changes. Type B change only by the more gradual process of antigenic drift.

DO ALL VIRUSES HAVE STRAINS?

- No. Only A subtypes and B viruses are characterised into strains.

- Strains only occur through antigenic drift.

HOW OFTEN DO FLU VIRUSES MUTATE?

- Frequently. They mutate whenever the small errors they make while replicating form a changed protein.

SO HOW WOULD H5N1 JUMP TO HUMANS?

- Once a human is infected with H5N1, the virus could potentially change its genetic makeup or combine with an existing human flu virus - through "re-assortment".

WHY HASN'T THIS HAPPENED ALREADY?

- It is still difficult for humans to catch bird flu because it has "receptor binding sites", which prefer to lock on to receptors in bird hosts, not humans.

- Some experts believe H5N1's receptor binding sites will adapt to human hosts to transmit easily among people, and spark a pandemic that could kill millions.

Sources: The World Health Organisation (www.who.int)

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