Human Influenza

Birds are the reservoir for all known influenza A subtypes

“There are four types of influenza viruses: A, B, C and D. Human influenza A and B viruses cause seasonal epidemics of disease (known as the flu season) almost every winter.  Influenza A viruses are the only influenza viruses known to cause flu pandemics, i.e., global epidemics of flu disease. A pandemic can occur when a new and very different influenza A virus emerges that both infects people and has the ability to spread efficiently between people.  Influenza type C infections generally cause mild illness and are not thought to cause human flu epidemics. Influenza D viruses primarily affect cattle and are not known to cause illness in people.  Influenza A viruses are classified by subtypes based on the properties of their hemagglutinin (H) and neuraminidase (N) surface proteins. There are 18 different HA subtypes and 11 different NA subtypes. Subtypes are named by combining the H and N numbers – e.g., A(H1N1), A(H3N2)”

How avian influenza passes to people

“Birds are the reservoir for all known influenza A subtypes; novel influenza viruses can emerge from birds and infect mammals including humans. Because swine are susceptible to infection with both avian and human influenza viruses, novel reassortant* influenza viruses can be generated in swine by reassortment of influenza viral segments leading to the “mixing vessel” theory. ”

“Birds are the reservoir for all known influenza A subtypes; novel influenza viruses can emerge from birds and infect mammals including humans. Because swine are susceptible to infection with both avian and human influenza viruses, novel reassortant* influenza viruses can be generated in swine by reassortment of influenza viral segments leading to the “mixing vessel” theory. ” 

Influenza can be passed from animals to humans directly (direct contact, airborne, aerosol, or bite) or indirectly (picked up from a contaminated environment e.g. water or via fomites -materials/ objects which can carry infection e.g. clothes/ tools).

“Infected birds shed avian influenza virus in their saliva, mucous and feces. Human infections with bird flu viruses can happen when enough virus gets into a person’s eyes, nose or mouth, or is inhaled. This can happen when virus is in the air (in droplets or possibly dust) and a person breathes it in, or when a person touches something that has virus on it then touches their mouth, eyes or nose. Rare human infections with some avian viruses have occurred most often after unprotected contact with infected birds or surfaces contaminated with avian influenza viruses. However, some infections have been identified where direct contact was not known to have occurred”.