First seen reshaping hapless humans in: Resident Evil 4
Though not as famous as Resident Evil's T-Virus (a.k.a. the rock star of viruses), effects of the Las Plagas parasite can make a T-Virus infection look like the sniffles. Discovered by the ancient Los Illuninados cult, Las Plagas was unearthed by Albert Wesker in his bid to return Umbrella Corporation to its former glory. It appears in the Resident Evil universe as a sentient parasite called Plaga that clings to a host's nervous system and warps them from within. Various forms (Blade Plagas, Tentacle Plagas, Crab Plagas, etc) can trigger obscene mutations, turning victims into nightmarish horrors. Horrors with awesome powers, mind you, but horrors all the same.
Treatment: Special medication can slow a Plaga's growth and painful radiation therapy has been shown to work on rare occasions. In both instances, the odds of making it to old age with a Plaga wrapped around your spinal chord are slim. Bright lights are known to hurt Plaga, so secluding yourself on a sunny island might be the best bet for everyone concerned.