A  B  C

Influenza viruses come in three types: A, B and C. How any one virus acquires its identification letter depends on differences in the matrix protein, nucleoprotein and response of antibodies to a particular strain. Influenza A is the most dangerous for humans, influenza C is the least dangerous.

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H & N

The H stands for one of the 16 different hemagglutinin proteins contained in a virus that allows it to penetrate a foreign body. The "N" stands for another protein in the genetic make-up of a flu virus called neuraminidase, of which there currently exist nine variations.