Some ailments obviously rise to the level of "disability" under the ADA. Others are more marginal. To help you make that decision, try to look at life from that employee's point of view. It's a common trial tactic used by plaintiffs' lawyers to help sway juries in ADA cases.
Such dramatizations often include professionally shot videotape highlighting the disability and "day in the life" of their clients. For example, a worker with a back injury may be shown being helped out of bed, and an employee on dialysis will be shown tethered to a blood-cleansing machine.
Recent case: James Heiko was a successful employee at Colombo Savings Bank until his kidneys failed. He needed a transplant and had to travel to a dialysis center and spend three hours hooked up to a machine. If he didn't undergo dialysis, he would die. Heiko rearranged his schedule by coming in early on dialysis days and managed to work a full week.
When he was passed over for a promotion, with the job going to a less qualified person, he shot off an ADA lawsuit. The bank refused to recognize his condition as a disability. Heiko's lawyers were able to show at trial how the disease altered his life dramatically. The appeals court sided with Heiko and was incredulous that the bank refused to see this as a qualified disability.
The ADA requires disabled employees to show their condition substantially impairs a major life function. Because eliminating waste from one's system is certainly a major life function, being hooked to a machine to eliminate that waste is a substantial impairment. (Heiko v. Colombo Savings Bank, No. 04-2046, 4th Cir., 2006)
Bottom line: Denying an obvious disability is a sure way to the courthouse. When in doubt, it's easier and cheaper to seek an accommodation, rather than turning a blind eye.