What determines animal to human or human to animal transmission of diseases?

How do pathogens act differently in humans compared to animals? Why, for example, are mosquitoes able to carry malaria without harming themselves? HIV/AIDS and rabies are diseases that can be transmitted from animals to humans, but are there any diseases that can be passed from human to animals?

pathogens have specific nutritional requirements and/or attachment sites that are present only in the host that they parasitize. these nutritional requirements and attachment sites are found in specific speciesand that renders that particular species susceptible to infection by the parasite. this is an example of species-related natural immunity.

to give you an example, african blacks are more resistant to plasmodium vivax (benign tertian malaria) infection because most of them lack the duffy factor in their red blood cells which serve as attachment sites for the parasite. cowpox virus infects cattle but we humans are resistant to it. we can however use attenuated cowpox virus particles as a vaccine against the deadly smallpox because cowpox and smallpox are somewhat related in structure but vary in terms of their host specificity.

there are diseases that could be passed from humans to animals. some good examples i could think of are the tapeworms and flukes. we harbor the mature forms of these parasites and through our excreta, the ova of these parasites could gain access to the habitats of their next host (the animals). examples of these are the fish tapeworm, beef and pork tapeworm, and the sheep liver fluke (infecting fish, cattle, pig, and snail respectively)

When a virus infects a cell it doesn't dig a hole into a cell wall. Rather, it binds to a specific site on the cell wall and tricks the cell into letting it in. So a human virus would not be able to bind to the same receptor on a mosquito cell and get into the cell and vice versa.

A virus will evolve to target one species or numerous related species. For example SARS has evolved to target birds. It can infect humans, but humans still have more immunity to it than birds do. Scientists are concerned, however, that after infecting a human, SARS could evolve into a species more virulent in humans.

we are genetically different than animals, and they have certain genes that enable them to be resistant to the diseases that they carry, without actually exemplifying the disease in their own phenotype. When they pass the genes onto humans...aka biting them, sucking blood in the case of mosquitoes, etc..they pass the gene onto us and our bodies are simply not immune to the diseases because our genetic makeup isnt the same as theirs. I am sure that human diseases could be passed on to animals if they were not already immune to them, but most humans with diseases dont run around biting animals ;)

I had heard of orangutans catching the flu from us, did a little reading ...
the poor things can catch polio, hepatitis,cholera, tb and something I have never heard of.