Feline Leukemia Virus
FeLV is a retrovirus, like HIV, and like HIV it can lie dormant for many years. As far as we can tell, Hermes was probably already infected when we first met him. It can be transmitted by grooming, biting, sharing a litter box or food/water (rarely), or nursing. Therefore, our other cat has been exposed, as has our neighbor's cat, and a cat we fostered while fostering Hermes. I hope none of them were infected. We'll be notifying our neighbors of the results soon, and have contacted the rescue program Hermes came from as well.
FeLV can apparently cause a variety of problems. It's the most common cause of cancer in cats. It also suppresses the immune system, leading to opportunistic infections. And it can impair the creation of new blood cells, causing anemia. The last issue appears to be what caused Hermes's death. An estimated .5% of all cats have persistent FeLV infection, while at least a third have antibodies against it (meaning they were exposed but successfully beat the virus). A vaccination is available (Hermes was actually vaccinated, which is why we think he got it before then - vaccination is useless if the cat is already infected). It's more common among city cats, since they're exposed to more cats, and especially common among feral cats (Hermes was caught at the site of a feral cat colony).
Information about FeLV: