Named for its prevalence in the little known town of Lyme Connecticut, Lyme disease is enemy number 1 on the Appalachian trail. Once the attrition rate begins to ween (north of the Mason Dixon), Lyme disease becomes one of the primary reasons hikers find themselves off-trail. The disease is a tick-born illness, one of the many, and can be extremely debilitating if not treated in a timely manner. The deer tick variety is the usual suspect for Lyme. This can be extremely problematic due to the pin-head size of these little vampires. 
Thus far, the majority of us have had a number of deer ticks on us. Generally they like to congregate around the warm/moist areas of the body. Waist bands, arm pits, groins, belly buttons, and hair lines are a few of the favorite hot spots for deer ticks. Often they can be seen scurrying across your legs or arms in search of a nice warm site to latch on, unlike the dog or wood ticks which will usually tap into you wherever they land. Fortunately for hikers, it takes at least 36 hours to contract Lyme disease. If a tick is found latched onto you within a day or so you can be confident that you have not gotten Lyme. That said, a daily (and very thorough) tick check is all one needs to avoid the sickness. 
If by chance you do happen to contract Lyme, the illness would show first as a red bulls-eye rash around the initial bite site. If still left unattended, the body will present additional bulls-eye rashes and you are now, for lack of a better phrase, shit outa luck. If you have let it get this bad, you are in for a long recovery. 
As a precautionary measure, many inkers have chosen to carry a course of doxycycline, the primary anti-biotic treatment for Lyme. As you might imagine, acquiring these drugs presents its own challenges. Without a current ailment, most doctors won't prescribe a course of anti-biotics. And even if they do, having something in your bag that you don't use every day will inevitably get wet. Your sleeping bag is usually your priority when is rains, then your food, then your clothes etc etc. Things like my beer coozy, my duct tape, and other things that get used on a now-and-again basis will inevitably get wet. I imagine pills would bloom into little popcorn looking pellets if they got wet.