If left untreated, HIV (human immunodeficiency virus) leads to AIDS. With modern treatment (cART), HIV is now a chronic, manageable infection: when the virus is fully suppressed, the person is no longer infectious ("U=U" — Undetectable = Untransmittable).
Prevention: Condoms, PrEP (preventive pill), PEP (emergency medication within 48–72 hours after possible exposure). Swiss resources: Aids-Hilfe Schweiz, Checkpoints, HIV clinics at university hospitals.