HIPAA (Health Insurance Portability and Accountability Act, 1996) regulates the use and disclosure of Protected Health Information (PHI) in the United States. When a Covered Entity (health plan, healthcare provider, healthcare clearinghouse) shares PHI with a vendor, the vendor becomes a Business Associate and must sign a Business Associate Agreement (BAA).
The BAA obligates the Business Associate to: implement administrative, physical, and technical safeguards; report security incidents and breaches; ensure subcontractors agree to the same restrictions; return or destroy PHI at contract end; and provide an accounting of disclosures.
Most SaaS vendors offer a HIPAA BAA on enterprise plans. Free or low-tier plans typically don't include it because the operational burden of HIPAA compliance is significant.