Farrakhan, 73, said he began suffering pain earlier this year similar to 1998, when the Nation of Islam leader was diagnosed with prostate cancer and later underwent surgery.
Doctors discovered an ulcer in Farrakhan's anal area during a visit to Cuba in March, Farrakhan said in a letter dated Sept. 11 and published in the Nation of Islam's "The Final Call" newspaper. He said he has since lost 35 pounds while battling "serious infection and inflammation."
Farrakhan said he will work hard to recover "because I do not believe my earthly work is done."
He said he asked his executive board to solve problems during his recovery, calling his absence a test that the Nation of Islam is stronger than any one individual and will live on "because it is rooted in an idea that comes directly from Allah (God) Himself."
Farrakhan compared the challenge to the situation in Cuba earlier this year, when Fidel Castro relinquished power due to illness.
"While many rejoiced — believing and thinking that if Fidel Castro and the Cuban Revolution expired they could move Cuba and the Revolution in a new direction — his absence from the helm only proved that Cuba will not fall apart over the absence or passing of their illustrious leader," Farrakhan wrote in the letter.
He also warned followers to be "ever watchful for any smart, crooked deceiver and hypocrite who would create confusion over my present condition."