A few years ago I was involved in a bread baking event for children from my parish before First Communion. The idea was that some adult volunteers would have about 12-16 children under their care at a big table in the school hall, and together we would mix and knead yeast bread. Afterwards the dough would be left to rise, and the children would head off to their next activity which was some sort of prayer service. When they returned they would be given their nicely baked individual loaf. (Volunteers had risen dough ready to go from the night before to facilitate this almost instant result).
Before we commenced, all the children were marched off to the bathroom to use the loo and wash their hands. Silly me, I thought since they were supervised they would all actually wash their hands and that the supervisor would actually supervise.
Anyway, we got going, measuring, talking about the basics of how the yeast would make the dough rise. The big bowl was passed around the table a few times and each child got a few turns poking it with the wooden spoon, and then I divided the dough ball into several small ones so they could break into smaller groups for kneading. It was at that point that I noticed a little girl with the filthiest pair of hands I had ever seen, dirt under the nails, smears on the fingers - ewwwwwww, too late to prevent her touching the dough, and I thought it would embarrass her if I sent her off to the bathroom so I swallowed hard, smiled, and cringed inwardly. The children had a blast with their very enthusiastic and unorthodox kneading and then off they skipped when the activity was over.
I was left with this very dubious bowl of kneaded dough, and since I had a few errands to run I put it in the car and was out for a few hours. When I got home the dough had risen beautifully. I thought, 'Feck it anyway, I'll bake it and see how it turns out - the heat will kill off anything microbic' - I tend to be an optimist...
The bread turned out gorgeous. It was the best loaf I have ever tasted. The crust was perfect. I suffered no ill effects.
The lesson is that a car on a sunny day is a great place to let dough rise.

(Also, I suspect a bit of dirt isn't really a problem, but I confess I often have to tell myself not to go at other people's children with a tissue, because snot everywhere is awful and children shouldn't have to go through life looking like urchins).