Yetiism is a disease caused by the Yeti pineal gland, which contains a concentrated form of the Yeti neurotoxin. According to Zoidberg: "It mimics the symptoms of hypermalaria exactly. Except for the part where, instead of dying, you turn into a Yeti!" Farnsworth had Yetiism from 2927 until 3011 (6ACV18).
Fry: [On seeing Farnsworth during the peak of his Yetiism.] My God. He's like some kinda believable Hulk!