You shouldn't spread diseases through sex.
Charles and Jill are about to have sex, but Charles has an STD, though he is not yet aware.
Charles wants to have sex with Jill.
Charles puts on a condom before having sex with Jill.
Charles has sex with Jill without using any kind of protection.
Charles needs medication for his STD, but does not pass the disease to Jill.
Charles passes his STD to Jill, and they both need medical treatment.