The two-term Atlanta-area congressman, who has appeared thin and frail in recent months, said in a statement that he has been undergoing aggressive treatment to contain the disease. He said the treatment is going well and that he has been free of the hepatitis C virus for almost a year.
Johnson's spokesman, Andy Phelan, said Johnson was first diagnosed in 1998 and is on his third round of treatment.
"My physician is encouraged by my response to treatment and expects complete success eradicating the virus," Johnson said.
Johnson, 55, said he doesn't know how he contracted the virus, which is spread through contact with infected blood. He said he plans to use his public profile to raise awareness of the disease, which infects some 4 million Americans.
"I want to send a strong message that a cure is possible but you must be tested and treated," the Lithonia, Ga., congressman said.
Johnson's frail appearance has stirred speculation on Capitol Hill about his health, but his office previously denied he was sick. In his statement, Johnson said the disease has not affected his ability to do his job.
According to the Centers for Disease Control, between one percent and five percent of people who contract chronic hepatitis C die from the consequences. Infection accounts for an estimated 8,000 to 10,000 U.S. deaths each year.