If a lot of people have used the porta potty then you could get some diseases such as HIV or other disease through contact with the seat of the porta potty if someone had those diseases and used the porta potty.
It's very unlikely that you'll get any diseases simply from sitting down on the porta potty seat but if you're worried and need to use the porta potty then you can prevent yourself from getting those diseases by just hovering over the porta potty seat.
That way you make no contact with the seat and cannot get any diseases from the seat.
Or you could put some toilet paper down on the toilet seat to help keep the disease and germs off your bottom.
I hate using porta potties myself and would prefer to pee or crap my pants instead but I've used them several times but never have gotten any diseases from those porta potties.