Fleas are small, irritating biting parasites, that are easily transmitted from host to host.
Fleas are caught by being in the vicinity of someone else with fleas for a reasonable length of time. They do not appear from nowhere on players randomly, the original sources are cats that rub against you or dogs. Stay away from them.
It is also possible to catch fleas as one of the punishments for playing the organ in the temple of Gufnork in Ankh-Morpork with a low skill level.
Fleas cause the victim to scratch, which (apart from being unsightly) will bring them out of hiding.
Fleas also cause regular minor injury of about 20 hp, whenever a message reads, "Ouch! Something bit you!" The damage can go unnoticed, but if something else reduces your health to a few hp and you are bitten by fleas, then they can kill you.
There are various ways to cure fleas: