--there are a number of pathogens--viral and bacterial--that can cause neuropathy by direct infiltration of nerve.

This website gives a good rundown of the possibilites--and there are a lot of them:

http://www.neuro.wustl.edu/neuromusc...her/infect.htm

Probably more often, though, infectious-related neuropathies are not the result of the infection directly, but result from an autoimmune molecular reaction. This occurs when the body fights off an invading pathogen, but the structure of the pathogen was sufficiently similar to that of some body tissue that the now activated immune system cannot differentiate that tissue from the pathogen and attacks it.

This mechanism is supected in many so-called "idiopathic" syndromes--it's hard to identify specific mediating antibodies as they may be unique to each person--and is thought to possibly underline a whole host of autoimmune conditions, including Type I diabetes, myasthenia gravis, and others.