Here's the reason I think some viruses affect cats and humans alike. Now, I'm not a biologist or a scientist, so disease transmission isn't my forte, so if someone has an explanation, please feel free to chime in.
My sister-in-law's cat became ill. Symptoms: Vomiting, diarrhea, general lethargy.
Vomitting lasted one day, diarrhea for 2 days, lethargy for 2 days. No treatment was given. Condition cleared on its own and the cat was back to full speed normal by day 3.
We all assumed she had eaten a bit of spoiled food, since there was a question as to whether a portion of rabbit had remained out on the counter too long.
Anyway, my sister-in-law had been over to our house to visit on the first day. She told us her cat had vomited several times in the morning. All the while she is playing with and petting our cats in the house.
Two days after her cat is completely better, the first of our 6 cats in the house gets sick. Same symptoms.
Within another day we have 4 sick cats. At this point my sister-in-law is sick as well. Same symptoms as the cats.
Another day goes by and my wife becomes ill. Same symptoms as the cats.
The other 2 cats are ill now too. The first 4 cats are nearly recovered.
Each cat and person who got sick, was better by day 3.
I'm the only one who didn't get sick. Once our house cats started getting ill, we took precautions not to bring this into our cattery. None of the cattery cats became ill.
Is there another reasonable explanation as to why this situation occured? Can't possibly be coincidence and it's not a case of food poisoning.