Generally, the Americans with Disabilities Act (ADA) looks at the effect a condition has on the person to decide whether that condition is a disability. A disability is defined as a physical or mental impairment that substantially limits a major life activity, such as walking, seeing, or taking care of oneself. Because courts were interpreting this language in a limited way that left a lot of people with serious conditions unprotected, Congress passed the Americans with Disabilities Act Amendments Act (ADAAA), which makes it clear that the term "disability" should be defined more broadly -- in favor of covering more people.
Among other things, the ADAAA includes major bodily functions in the list of major life activities, so impairments that are not yet evident on the surface but are causing serious problems within the body (such as the early stages of HIV and cancer) qualify as disabilities. The law also provides that impairments that are in remission but would affect major bodily activities when active are disabilities under the law. For more information on the ADA and the ADAAA, see Nolo's articles Disability Discrimination in the Workplace: An Overview of the ADA and ADA Amendments: More Protections Against Disability Discrimination.