CHICAGO (AP) — Potentially dangerous coronavirus misinformation has spread from continent to continent like the pandemic itself, forcing the world's largest tech companies to take unprecedented action to protect public health.

Facebook, Google and others have begun using algorithms, new rules and factual warnings to knock down harmful coronavirus conspiracy theories, questionable ads and unproven remedies that regularly crop up on their services — and which could jeopardize lives.

Health officials, critics and others who have long implored the tech companies to step up their response to viral...