less infection in humans
Human infections have been detected relatively rarely – fewer than 900 have been documented globally over several decades – but about half of those infected individuals died. The good news about H5N1 for humans is that it does not currently spread from person to person. Most people who contract H5N1 get it from direct contact with infected poultry – especially chickens, turkeys, and ducks, which are often raised in large numbers on large commercial farms.
There are only a few examples of human-to-human spread. Since the infection does not spread from human to human, and direct infection of humans from infected birds is still relatively rare, H5N1 has not yet emerged as a human pandemic or pandemic. Why is there suddenly so much attention being paid to this outbreak? The first reason bird flu is getting so much attention right now is that H5N1 is currently causing the largest “bird pandemic” ever recorded. A certain viral variant that originated in 2020, called H5N1 2.3.4.4b, is driving the outbreak.
in which animals
In farm poultry flocks, if some birds are found to be infected with H5N1, the entire flock is culled, regardless of symptoms or infection status. High prices for eggs and poultry meat in the US are one result. The Biden administration is considering vaccinating farmed poultry flocks, but the process could be complicated. Another reason for the increased attention is that H5N1 is now infecting more bird and mammal species than ever before. The virus has been detected in a wide range of wild birds and various mammals, including badgers, black bears, bobcats, coyotes, ferrets, fisher cats, foxes, leopards, opossums, pigs, skunks, and sea lions.
also spread in sea lion
It has not been definitively confirmed whether sea lions are spreading the virus to each other or contracting it from birds or H5N1-infected water. The key question here is: If H5N1 can spread to mink and possibly sea lion, why not humans? We are also mammals. It is true that like chickens on poultry farms, farm minks are also kept together in large numbers, which increases the possibility of its spread. In contrast, humans experience influenza as a respiratory infection and spread it through breathing and coughing. Over the centuries, some of these avian influenza viruses have passed from birds to humans and other mammalian species, although this occurs less frequently. 
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Most intestinal infections
Avian flu viruses have evolved to infect cells of the intestinal tract, while human flu viruses have evolved to infect cells of the respiratory tract. However, sometimes a flu virus can acquire mutations that allow it to infect cells in a different part of the body. The cells that influenza infects are partly determined by the specific receptor it binds to. Receptors are molecules on the surface of host cells that a virus uses to enter cells. Once viruses are in cells, they may be able to produce copies of themselves, at which point infection has occurred.
main concern
Both human and bird influenza viruses use receptors called sialic acids that are common on the surfaces of cells. Bird influenza viruses, such as H5N1, use a variant called A2,3-linked sialic acid, while human flu viruses use A2,6-linked sialic acid. Thus, to become efficient at infecting humans, H5N1 would need to mutate to use A2,6-linked sialic acid as its receptor. This is a concern because studies have shown that only one or two mutations in the viral genome are sufficient to switch receptor binding from A2,3-linked sialic acid to human A2,6-linked sialic acid.
