Shug's illness is never specified. Celie hears various rumors. Some say Shug has "two berkulosis" (tuberculosis) or a "nasty woman disease," which could be in reference to some sort of venereal disease. Celie is curious to know more but doesn't ask. She summarizes the gossip as follows:
Shug Avery sick and nobody in this town want to take the Queen Honeybee in. Her mammy say She told her so. Her pappy say, Tramp. A woman at church say she dying—maybe two berkulosis or some kind of nasty woman disease. What? I want to ast, but don't.
When Shug arrives in a wagon, she is sick and weak but nevertheless dresses to the hilt in a red dress with black beads and a feathered black hat. As Celie nurses her, she decides,
Ain't nothing wrong with Shug Avery. She just sick. Sicker than anybody I ever seen.
Celie also says that although she is very sick, Shug will recover because she is "evil." Shug has a mean streak and assertiveness of character that ensures she will fight hard to stay alive.
Shug's sickness is a plot device that allows Celie to get to know this vibrant and dominant woman in a guise where the two can be equals because of Shug's weakened state and dependence on Celie's care. Shug's enforced isolation and weakness allow the women to become aware of each other both emotionally and physically. Celie, for example, has to bathe Shug and begins to awaken to the nature of her own sexual desires. The intimacy the two share lays the groundwork for the love affair that will follow.