"There are no data to indicate that snake parasites or diseases regulate wild populations. In captive individuals, fungal infections and respiratory problems can be substantial (Narcisse den individuals have been observed with fungal infections). A parasitic fluke often causes wild snakes to lose the ends of their tails and may be responsible for breeding failures. Its incidence varies considerably from year to year and appears related to the fall condition of snakes (poor condition is associated with high fluke infestations)."

It's also cited in the academic literature, but I don't have access to the online journals.

http://www.naturenorth.com/spring/creature/garter/Fgarter.html

We ran into this a lot with a field survey of the only documented population of black racers in Maine. Many of the older animals were also missing their tail tips. Garter and Water snakes are notorious for high numbers of flukes. There is no retained shed on the tail of the albino.