I would guess that it happens but not very frequently. And when it does happen, odds are that people don't associate it with the reptiles and chalk it up to the infamous "stomach virus". In healthy people it is not particularly dangerous and will pass in a couple days. Of course like any illness with children, the elderly and those who are immuno-compromised it can be deadly. I wash my hands before and after I touch any reptile. I keep anti bacterial gel in the snake room as well. Don't wash anything that has come in contact with reptiles in the same sink where you put your dishes. I got a "stomach virus" a couple months ago. I suppose it could have been Salmonella...