What you're talking about here is tropism - what host a virus can infect, and not pathogenicity - whether or not it can make you sick and to what degree. They are related because the virus needs to be able to infect you in order to make you sick, but tropism is just one part of a much bigger picture.
Influenza can infect people, birds, pigs, cows...even whales!
Notice next to each one it has an indication of H or N with a number? That's the type of hemagglutinin (H) or neuraminidase (N) - two proteins on the surface of the fly virus that are important to the virus getting into a target cell. Notice the duck in the middle says H1-15 and N1-9? Yeah, waterfowl are the native hosts for influenza and carry all the different types in all the combinations. All the other animals are more limited. We name a virus by what H and what N it has and that's how we get H1N1, or H5N1. Hemagglutinin is really important to how the virus attaches to and enters a cell in your body. Despite it being the same protein in all different flu viruses, each number is slightly different. Each of these Hs binds to a modification on protein in your lungs called sialic acid. And... different types of animals have different sialic acids in different places in their lungs. So, for an animal to even get infected with a version of the flu it's sialic acid needs to match the virus's H.
H5 is one of the ones that scares us because it CAN infect humans, and when it does it's less than great. However, it seems like it's only possible to get an H5 virus directly from a bird and not from another human. (This is what public health people will monitor for - human to human transmission). The reason for this is where the right sialic acid for H5 is in our lungs... it's WAAAY down the bottom and you have to breathe in a lot of virus to get it there and start the infection, so it's not easy to transmit and probably doesn't spread to the rest of the lung where it could make enough of itself to infect your neighbors.
BONUS edit - there are actually more viruses than you might expect with a broad host range. Poxviruses can get around, you can share SARS-CoV-2 (COVID) with your dog and the deer in your yard, and arboviruses HAVE TO infect both insects and humans to be passed along successfully.
ProfKlase t1_j7sl4ev wrote
Reply to Is the H5N1 bird flu unusually zoonotic compared to other viruses? by PURELY_TO_VOTE
Oh, this is a good one!
What you're talking about here is tropism - what host a virus can infect, and not pathogenicity - whether or not it can make you sick and to what degree. They are related because the virus needs to be able to infect you in order to make you sick, but tropism is just one part of a much bigger picture.
Flu is even crazier than just sea lions, birds and humans. Check out this picture: https://www.semanticscholar.org/paper/Sialobiology-of-influenza%3A-molecular-mechanism-of-Suzuki/4bdb9bbce24fcc2dba5a37a65f5e33052119e4be/figure/0
Influenza can infect people, birds, pigs, cows...even whales!
Notice next to each one it has an indication of H or N with a number? That's the type of hemagglutinin (H) or neuraminidase (N) - two proteins on the surface of the fly virus that are important to the virus getting into a target cell. Notice the duck in the middle says H1-15 and N1-9? Yeah, waterfowl are the native hosts for influenza and carry all the different types in all the combinations. All the other animals are more limited. We name a virus by what H and what N it has and that's how we get H1N1, or H5N1. Hemagglutinin is really important to how the virus attaches to and enters a cell in your body. Despite it being the same protein in all different flu viruses, each number is slightly different. Each of these Hs binds to a modification on protein in your lungs called sialic acid. And... different types of animals have different sialic acids in different places in their lungs. So, for an animal to even get infected with a version of the flu it's sialic acid needs to match the virus's H.
H5 is one of the ones that scares us because it CAN infect humans, and when it does it's less than great. However, it seems like it's only possible to get an H5 virus directly from a bird and not from another human. (This is what public health people will monitor for - human to human transmission). The reason for this is where the right sialic acid for H5 is in our lungs... it's WAAAY down the bottom and you have to breathe in a lot of virus to get it there and start the infection, so it's not easy to transmit and probably doesn't spread to the rest of the lung where it could make enough of itself to infect your neighbors.
Vincent Racaniello has a good simple writeup on this: https://www.virology.ws/2009/05/05/influenza-virus-attachment-to-cells-role-of-different-sialic-acids/
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BONUS edit - there are actually more viruses than you might expect with a broad host range. Poxviruses can get around, you can share SARS-CoV-2 (COVID) with your dog and the deer in your yard, and arboviruses HAVE TO infect both insects and humans to be passed along successfully.