Amnesty International has appealed to the government of Ivory Coast to grant urgent independent medical care to jailed opposition leader Alain Lobognon.
Lobognon, who is also a deputy, was arrested on December 24 last year.
Amnesty said he was suffering from high blood pressure and had undergone surgery last year.
He was experiencing symptoms of malaria and had stayed in his bed for four days, the group wrote Friday in an open letter to Ivorian Justice Minister Sansan Kambile.
"According to information shared in a letter by his lawyer and wife, his current poor health is due to poor detention conditions," Amnesty wrote.
His cell was infested with mosquitoes and insects, the group added.
"As an opposition leader, Alain Lobognon fears for his life and would like to have access to his own independent doctor."
Lobognon is an ally of former rebel chief Guillaume Soro who is a candidate for the presidential election due in October.
Lobognon was arrested just after Soro had to cancel his return to the country from exile in France when the government issued an arrest warrant for him, accusing him of attempted insurrection.
Another 16 members of Soro's party were also arrested in late December.
Amnesty said it considered the timing of these arrests "highly suspicious", adding: "The charges appeared politically motivated."