Anderson was arguing with Dugan about an anti-immigration video when he saw Dugan scratching red marks on his arm. He insisted Dugan go to the hospital. He was with Dugan as often as possible as Dugan kept working even when in the hospital.
However, Anderson learned that Dugan had leaked the video. He publicly fired Dugan days before the election. However, this was part of Dugan's plan - the video was meant to inflame pro-immigration sentiment and Dugan would be seen as the bad guy and Senator Anderson the good guy.
Gregory House was watching Anderson's announcement when he noticed Anderson was sweating heavily. He realized that Anderson had hepatitis C and that Dugan probably had it too even though he had tested negative. He first accused Dugan of having sex with Anderson, but Dugan soon admitted it was most likely the infection was transmitted because they shared cocaine.
However, they couldn't give Dugan a controversial treatment, infecting him with Hepatitis A in order to set off an immune reaction, as long as Dugan tested negative. House convinced Anderson to have a test for the condition and tested under Dugan's name to get a false positive result for Dugan while keeping Anderson's anonymity.