Influenza (flu) is caused by the influenza virus.
This virus has a seasonal pattern of appearance, hence is called seasonal influenza.
There are three types of seasonal influenza viruses: A, B and C.
Type A may infect multiple species- Humans, pigs (Swine), birds (Avian).
Type B only infects humans.
Type C may infect humans and pigs.
Type A may have sub-types depending upon the combination of two proteins, namely Haemagglutinin (H) and Neuraminidase (N). These proteins may have different numbers:
H: 1 to 17
N: 1 to 10
The combination of numbers determines the name of the virus. Thus, we have H1N1, H1N2, etc.
From a public health perspective, Type A is more important as it can cause epidemics and pandemics in addition to seasonal flu.
Type B may cause seasonal flu and epidemics, but not pandemics.
Type C only causes mild respiratory symptoms, not epidemics or pandemics.
The name assigned to each flu virus indicates the characteristics mentioned above. The present flu virus in India is A(H1N1)pdm09 virus. This indicates that
It is a Type A virus
It is a virus with H1 and N1 proteins in combination
The virus is the same that resulted in the influenza pandemic of 2009.