Being one of the "references" listed (thanks quickdtoo) I have to say that Permethrin is an effective way to prevent some parasites and biting insects from ruining your day. Kill them with Permethrin; don't just run them off with repellent.
Preventing illness from bugs in the woods is a tough topic. Most of us have been bitten, stuck, probed, sucked, stung, and a host of other insect unpleasantries. What happens after the "taste test" is generally NOTHING, unless one is alergic to say bees and wasps.
Listing human parasite vectors indicates there is an overwhelming Army of them. Each brings a different set of circumstances (this list is incomplete):
Ants, spiders, fleas; horse flies, yellow flies, biting house flies; [previously named bugs] chiggers and deer ticks; wasps, bees, hornets; mosquitoes, No-See-Ums (tiny intense stinging bugs/flies suggested as a vector of CWD); leeches, ring worm, round worm, pin worm, hook worm, liver fluke, tape worm; microscopic guys like protozoans (Giardia, amoebic dysentary, malaria - which is rare in the U.S.A.).
Human parasites are everywhere. It is easy to become infected through insect bites, animal feces, walking barefoot, handling raw meat and fish, eating contaminated raw fruits and vegetables, drinking contaminated water, inhaling contaminated dust (parasitic eggs or cysts). Fortunately many of us have immunities from past contact (to some of the microscopic guys anyway).
Knowing the "hazards" is not going to keep you from getting a few once in a while. Most illnesses are unpleasant but recovery is full. Only a few can kill you or leave a lasting impression.
Get to know the ones that can leave you with a "lasting impression". The rest should not keep you from the woods.