28 CFR 35.139: Direct threat.

(a) This part does not require a public entity to permit an individual to participate in or benefit from the services, programs, or activities of that public entity when that individual poses a direct threat to the health or safety of others.

(b) In determining whether an individual poses a direct threat to the health or safety of others, a public entity must make an individualized assessment, based on reasonable judgment that relies on current medical knowledge or on the best available objective evidence, to ascertain: the nature, duration, and severity of the risk; the probability that the potential injury will actually occur; and whether reasonable modifications of policies, practices, or procedures or the provision of auxiliary aids or services will mitigate the risk.