Lions, Tigers, Bears...
and those Annoying Bloodsuckers

There are real hazards in the back country.

Mosquitoes

I typically use a OR headnet and rely on my clothing which have been treated with Permethrin. In my opinion, the best way to prevent being bit is to use physical barriers like headnets and to use is DEET based repellent.  See Mosquitoes and Mosquito Repellents and DEET-based insect repellents: safety implications, and EPA's list of active ingredients for more information. Recently Picaridin has entered the US market. Picaridin has a number of advantages over DEET: it doesn't destroy plastics, it doesn't smell bad, and doesn't leave that icky feeling. From the studies I have read Picaridin is slightly less effective than similar concentrations of DEET. I have found that 8% Picaridin is effective against modest mosquito pressure for a few hours but not effective in heavy mosquito areas, and 30% worked well the one time I used it under medium mosquito pressure... I haven't yet used it when the mosquito pressure is very high. The safe bet is  time released 33% DEET such as Ultrathon or something with even more DEET. I have tried a number of the natural insect repellents. Lemon Eucalyptus seems to have been the most effective. In locations which have only minor mosquito problems they can be effective, but if you are somewhere with lots of mosquitoes (say in the Sierras when the snow is melting, Canada during the summer, etc), you will get eaten alive if you use "natural" repellents.

Ticks

Start with prevention.  Spray your cloths with Permethrin and use DEET based repellent if you are in a location which has a lot of ticks. You should remove ticks by gently pulling them off with a pair of tweezers or using one of the special tick removing devices which are designed to lift ticks off your skin (such as the Pro Tick Remedy).  Using  hot match, Vaseline, nail polish is extremely counter production.  Not only do these techniques risk killing the tick and leaving part of the tick in the victim, but ticks will often expel their stomach contents when distressed, the very thing you want to avoid.  There is a belief that Lyme disease isn't transmitted for 24 hours, so make sure you do a tick check each night.  If you have been bit by a deer tick, check with a doctor.  There is now an anti-biotic treatment which is more than 80% effective. I collected some info about tick removal tools from backpacker.com forum. Of course, treating clothing with Permethrin helps keep ticks away.

Snakes

In an average year less than 6 people died from snake bites in USA.  More people died from lighting strikes each year.  Most snake bites are the result of young males putting their limbs in locations that would violate basic common sense. So if you are hiking in US, don't get freaked out about snakes. Making incisions and above the bite is ineffective, and now you have a wound which must be tended. The only semi effective treatment in the field is Sawyer's Extractor which will remove ~35% of the venom if used within three minutes.  Even the extractor is being questioned.  These days the best recommendation to to extract people for medical treatment if possible, and if not keep them calm and well hydrated and be prepared to sit around for 2-3 days while they are really sick. http://www.aafp.org/afp/20020401/1367.html

Bears

Keeping you safe

Keep food safe

Fatal attacks by American black bear on people: 19002009