I would imagine the question of disease comes under the umbrella of trust. You have to trust that your partner will use protection with the other people they see. If you don't trust them to do that then that's another reason you might not be suited to an open relationship but again, it works for some people.
Sleeping with more than one person doesn't automatically equal the spread of nasties.
Sleeping with more than one person doesn't automatically equal the spread of nasties.