Dr. Sidney Curtis is one of the leading immunologists in the United States. He is an attending physician at the Mayo Clinic and is the author of Curtis on Immunology. He was recruited by the CIA to treat John in the Season 4 episode Need to Know. He was portrayed by actor Holmes Osborne.
The CIA also recruited Gregory House on the case and he was introduced to Curtis by Dr. Samira Terzi. House decided to get off on a bad foot by telling Curtis he recognized him from his book, then telling him that he used it to prop up one of the legs on his piano. House also blew off Curtis's suggestion that John was suffering from horse chestnut poisoning. However, Dr. Terzi went with Curtis's suggestion that John was suffering from radiation sickness.
However, House thought John had pancreatitis and, behind the back of Curtis and Terzi, started treating him for it. When John started getting better, House revealed his deception. Curtis was angry, and became angrier when John took a turn for the worse. However, Dr. Terzi went with House's idea that John had Waldenström's macroglobulinemia and started chemotherapy.
However, when John's hair started falling out well before the chemotherapy would have caused it, House had to admit radiation sickness was probably the right diagnosis. Curtis was furious. However, House had the last laugh when he realized John had selenium poisoning.
As they left, Curtis further denigrated House's recklessness, but Dr. Terzi supported House, reminding Curtis that House had turned out to be right in the end.