They are just the most godawful things ever.
Well. While they are rather horrible I can think of some that are worse. I have always felt fortunate to hike in an area that is rather benign in these issues. We do have ticks though I believe lime disease generally has a 36 hour time period from the time of bite till the actual infection becomes a problem. We do have mosquitos that fortunately don't present as many problems as they do elsewhere. We do have bears, while opportunistic they are small teddy bears that run off compared to say a grizzly or brown bear. We have wasps, yellow jackets, snakes and yes even scorpions, but still we have it pretty tame here.
My worse encounter to date was on the Carettera Austral or southern highway of Chile. I was in Parque Queulat and tried to get off trail to camp, ending up in a birdcage of bamboo. So after ten minutes down and 1 plus hours to get back to the trail I sleep on the trail and woke to find an number of leeches had found their new home on me. None of this compares to what folks farther north on the equator have.
My first trip into Bolivia was one of shock when I read the number and scope of possibilities I might encounter. Much of Bolivia is owned by the indigenous Indians as it was returned to them in the 50s from the conquistadors families whom had been granted much of the land under the Spanish. So your doing more trekking between villages than hiking. Many more remote places require you to also use camiones or the local transport, which is very likely a weekly affair of a flatbed truck with side rails to hold everything in. And I mean everything and anything. The point being your far from any major medical services.
Here's a list from lonely planet of the possible issues, though I've included some issues that are more than just insects.
Altitude sickness or Soroche
Acute mountain sickness - more severe than above.
Then we get to the infectious stuff
Rabies- Many folks have dogs. I got the first three injections as it buys you more time.
Then we get to the good stuff. The insects.
Chargas Disease - Listed as little possibility, but if trekking you try to stay out of thatched shelter. The Vinchuca beetle or the assassin bug transmits a parasite through its bite. The parasite causes hardening and constriction of blood vessels swelling organs. Fatal over many years period with no cure, though quick medical attention is necessary. They list 25 % of the population have this.
Leishmaniasis - Another one worth mention. Amazonian sand fly transmits this protozoan. Could lead to death by gangrene.
Edit: I was wondering about the Chargas Disease and read some more. There are other references that say the parasite isn't spread by the bite of the beetle. It may be that the beetle sucks so much blood that it needs to defecate and the parasite actually lives in the excrement of the beetle. So it may be worse than I'd thought if that's possible.
Well. You get the picture. We really live in a paradise. While some of this stuff is a remote possibility, the northwest has pests, but that is about as bad as it gets.
For what its worth.