leech

nouns

carnivorous or bloodsucking aquatic or terrestrial worms typically having a sucker at each end
bloodsucker, hirudinean, leech
a follower who hangs around a host (without benefit to the host) in hope of gain or advantage
leech, parasite, sponge, sponger

verbs

draw blood
"In the old days, doctors routinely bled patients as part of the treatment"
bleed, leech, phlebotomise, phlebotomize