Bedbugs and lice and pinworms: how do they do it?

There's a question that has been tickling the back of my brain lo these many years of parasite infestations: How do bedbugs and lice annex a new location and proliferate so rapidly? After all, it takes not one but two to procreate. So two individual lice, one male and one female, must jump to a new head simultaneously and find each other in a forest of hair in order to begin an infestation. Or a single pregnant female must make the leap—but for proliferation to ensue in that scenario, brothers and sisters would have to mate. And isn't there a law of nature against that?

Ditto with bedbugs. There's rampant fear that a friend with an infestation will contaminate your home by transporting them in on shoes or clothing and shedding them on your carpet. But ... what are the odds someone would track in a male and female simultaneously, or a pregnant female?

I don't even want to think about pinworms long enough to formulate a question to express my confusion about them.