On February 29 of this year, I contracted–somehow–an infectious disease that immediately manifested itself with symptoms of slight dizziness, nausea, vomiting, and lack of appetite. Within a week these symptoms disappeared, and my left calf became swollen, reddened, and scabby, and started oozing water and having shooting mini-pains within the skin. My doctor prescribed antibiotics for me, but eventually had me stay in a nearby hospital for five days, while the people there took blood samples and tissue cultures.
Since I left, those symptoms had faded, and I’m off the antibiotics. A lawyer, who, like me, is in his early 60s, claims to have similar symptoms (though nowhere near so severe) and says he got them from being in my room at home, where I have my computer and printer. I use this hardware to prepare legal documents the lawyer wants.
Lately he has refused to come into the room, claiming that he got infected from the room itself. The doctor told me today that the infection, as decribed in the lab report the hospital sent him, cannot be communicated that way, but the lawyer refuses to believe it.
Is he just being pig-headed?
I’m not a doctor, but the first episode sounds a lot like food poisoning to me. That is an “infectious disease” in that it’s caused by a microbe of some sort, but it’s mostly acquired from food or from direct contact with someone who’s currently sick. If the lawyer ate the same food, or if you weren’t good about washing up after the vomiting and diarrhea, he could have gotten the bug from you - when did his symptoms start?
The leg problem was probably completely unrelated. An infection from a scratch, possibly MRSA considering the hospitalization. Did the lawyer also get a skin infection? Unless you were wiping your scabby leg all over things he was going to be touching, then I doubt he caught that from you.
And hey, if he doesn’t want to come into your room, enjoy your quiet space. I know I would. I’d probably cough now and then to discourage him from coming in while I’m busy. If he needs to come into the room but is being a baby about getting sick, maybe let him see you wiping down hard surfaces with some Lysol wipes.
You, OTOH, may want to stay from him, so you don’t catch the crazy. 
(Seriously, your doctor knows the exact bug in question and is in the best position to know its particular means of transmission. If your lawyer does not want to believe that expert who has the actual information in front of them in preference to blaming you and your house, there is not much you can do.)
Antigen, you have started me thinking.
The Monday evening before (Feb. 29, of course, was a Wednesday) I ate a hearty ravioli dinner at a small Italian diner in San Pedro. I liked the food. I was quick to blame what happened Wednesday on it; in any case, the doctor said that in no wise could the symptoms (on the leg) be caused by food; he suggested, for example, a spider bite. However, we had had exterminators spray our place (a mobile home) months before, effectively ridding the dwelling of termites, and ants, that had moved in. The little house spiders that are still with us seem to be immune to the insecticide–after all, they are not insects–and they seem to have a live-and-let-live attitude; they don’t bite humans and transmit diseases to them. OK, maybe the Italian food made me toss my cookies that Wednesday, but the origin of the symptoms my left calf suffered is still a mystery.
They don’t need to “transmit diseases”. All it takes is a break in the skin, a couple of bacteria, and bad luck to end up with a raging infection.