Short answer
Blood-borne HIV transmission is narrow and preventable: shared injection equipment dominates real-world cases, occupational needlesticks get structured PEP protocols (transmission around 0.3% per injury from a positive source. Low but protocol-worthy), and the screened US blood supply approaches zero risk.
Casual blood contact, mosquitoes, dried blood, shared razors in practice, sterile tattoos, fights without blood exchange, transmits nothing.
Each route below gets its prevention matched to its mechanism.
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See Tests & Prices →Injection equipment route. The dominant real-world pathway
Sharing needles, syringes, cookers, cotton/filters, and rinse water transmits efficiently through direct blood exchange. Microscopic residue suffices.
Syringe service programs cut HIV (and hepatitis) transmission dramatically while linking users to care and treatment, without requiring abstinence.
Never share any item; use new sterile supplies every time; seek PEP within 72 hours after high-risk shares.
Full harm-reduction detail at HIV Risk from Needles and Injection Equipment.
Occupational exposures. Low risk, strict protocol
- Needlesticks from HIV-positive sources (roughly 0.3% transmission per injury. The figure guiding PEP decisions, not reassurance to skip them).
- Mucous-membrane splashes (lower risk, still evaluated).
- Bites with blood exchange (edge cases. Intact-skin bites carry no risk).
Every occupational exposure follows the same sequence: immediate washing (soap and water, no squeezing or bleach), rapid PEP evaluation (first doses within hours), source testing where available, and baseline plus window follow-up testing for the exposed worker.
Healthcare facilities run these protocols continuously. Report immediately, day or night.
Transfusion and transplant safety. Effectively historical risk
Donor deferral screening (behavioral and medical history filters), antibody plus nucleic-acid testing of every donation (closing window-period donations to near zero), and lookback systems (tracing prior donations when donors later seroconvert) together make transfusion HIV effectively historical in the US. Residual risk vanishing and continuously monitored.
Tissue and organ pathways mirror these safeguards with parallel testing and deferral systems.
Transfusion decisions proceed on their medical merits; HIV is not a reason to decline needed blood products.
Non-routes. Fears retired permanently
- Mosquitoes (no mechanical transmission: HIV cannot replicate in insects).
- Dried blood (virus dies rapidly on drying. Intact skin plus dried blood equals zero risk).
- Shared razors and toothbrushes in practice (theoretical only, zero epidemiologic cases).
- Tattoos and piercings with sterile equipment (licensed shops with single-use needles. Verify, then proceed).
- Fights without blood exchange (intact-skin contact, however vigorous, transmits nothing).
When to seek care
Immediately for PEP after qualifying blood exposures, for syringe-service and addiction-treatment linkage, and for baseline plus window HIV testing after any blood exposure.