Things I need:
A fungus that can infect healthy people, but cause minimal to no symptoms until the patient dies of something else (the fungus will not kill you, except perhaps in very rare cases). I'm ok with, eg, something kind of like a cold or whatever upon initial infection, as long as it goes away after a week or so, and with the resource drain you'd expect from any parasite (eg the host has to eat more), but not any symptom that would be a significant nuisance to a well-fed person.
For same to be relatively easy to transmit by fluid-to-fluid contact (eg bites, kissing, sex), but relatively difficult to spread by any other method (eg coughing)
For same to be capable of very rapid growth, under at least some circumstances (eg I need a newly infected and killed person to be contagious within a few minutes)
For same to, in essence, turn the body into a meat puppet when the host dies.
Problem: How do I get such a weird fungus, in general?
Solution: Years of radiation and chemotherapy. Patient zero caught the progenitor fungus as a kid, then had several bouts of lymphoma, and the fungus managed to hang on through all of them, evolving rapidly
P: Why isn't the fungus spread by air?
S: Swimming spores. Which may mean it needs to be a chitrid fungus. Which means I need to figure out if there's any chitrid fungus that ever infects humans.
P: How does the fungus "know" the body has died?
S: I am thinking of going with something like extremely low blood oxygen, but I'm not sure if there's a better trigger.
P: How does the fungus spread so quickly when the host dies?
S: I'm thinking something along the lines of... the fungus makes more spores than it releases. So, as long as someone has been infected for a while, their fungal mass has a lot of spores that can be released more or less explosively when the fungus senses death. Which means that a fresh zombie is more infectious than either a live host, or a zombie that's been dead for a while.
P: Why doesn't the body kick out the fungus?
S: This is the trickiest one. My initial thought was that the fungus, basically, managed to sneak in to the brain side of the blood-brain barrier, but then there's the question of why the body isn't reacting to the spores that the fungus is giving off. I still want the main body of the fungus in the brain (so it's in place to take over when the body dies), but I'm thinking of going with some kind of handwave along the lines of "manages to cloak itself with surface proteins from something that's supposed to be in the body" But I'm not sure if that's going to overstrain anyone's disbelief suspenders. If you have any better ideas, I'd love to hear them.
P: How is a fungus capable of complex behaviors like walking?
S: The fungus, in essence, mimics the pattern of its host's brain. Which means it has at least some of the host's memories and thought patterns.
Any problems you think I missed? Any problems with the solutions to the problems? Any suggestions for better solutions? Any other thoughts?