The Americans with Disabilities Act (ADA) protects people from discrimination on the basis of disability.
Under the ADA, temporary impairments don't qualify as disabilities. Conditions that last only a few days or weeks and having no long-term or permanent effects on the person's health will not qualify. Colds, the flu, broken bones, and sprains generally won't qualify as disabilities, as long as they don't have serious, long-term consequences. Pregnancy also doesn't count as a disability.
However, a short-term illness or other condition that has long-term complications may qualify as a disability. For example, if an employee breaks his arm and must be in a cast for six weeks, that generally wouldn't be a disability. However, if the employee's arm doesn't heal properly, and the employee is permanently unable to use it for more than a few minutes without pain, that would likely be a disability. Similarly, if an employee suffers a mild concussion in a car accident but recovers fully within a few days, it probably doesn't qualify as a disability. If, however, the employee suffers brain damage as a result of the accident, that would generally qualify as a disability.
Conditions that are permanent but result in only temporary periods of actual disability also qualify as disabilities. For example, a person with epilepsy generally will be found to have a disability, even though she rarely has seizures, and the few seizures she does have last for no more than a minute or two. In this situation, the condition is always present (and so not short term), even though its effects come and go.