HIV passes through only a few fluids — blood, semen and pre-seminal fluid, vaginal fluid, rectal fluid, and breast milk — and only when they reach a mucous membrane, damaged tissue, or the bloodstream[1]. In practice that means condomless anal or vaginal sex, shared injection equipment, and pregnancy, birth, or breastfeeding[5]. Someone on treatment with an undetectable viral load cannot pass HIV on sexually[6].
Route 1: sexual contact
Sex is the dominant route in the United States — CDC data indicate sexual contact accounts for at least 89% of new HIV diagnoses[5]. Either partner can acquire HIV during anal or vaginal sex, but the receptive partner faces higher risk because the rectal and vaginal linings are mucous membranes rich in the immune cells HIV targets[1][10].
Per 10,000 exposures with a partner who has a detectable viral load, CDC estimates receptive anal sex at 138, insertive anal at 11, receptive vaginal at 8, and insertive vaginal at 4[3]. Framed as odds, receptive anal sex is about 1 in 72 and receptive vaginal sex about 1 in 1,234 per act[9]. Oral sex is far lower risk but not zero; risk depends on viral load, oral health, and untreated STIs[10]. HIV can also be transmitted by sharing sex toys, which CDC classifies as negligible risk[3].
Route 2: shared injection equipment
Used needles, syringes, cookers, cotton, and rinse water can hold another person's blood, which is why sharing any of it — not just needles — carries risk[1][4]. CDC estimates 63 transmissions per 10,000 needle-sharing exposures, or roughly a 1-in-160 chance each time equipment is shared with someone who has HIV[3][4]. NIH notes that is nearly eight times the per-act odds for vaginal sex[5].
Needles used for tattoos and piercings carry the same principle when equipment is reused[5]. One honest caveat: CDC says it is unknown how much an undetectable viral load reduces transmission through shared injection equipment — it likely helps, but the amount isn't quantified[4].
Route 3: pregnancy, birth, and breastfeeding
HIV can pass to a baby during pregnancy, childbirth, and breastfeeding — historically the most common way children acquire HIV[1]. Without any preventive measures, aidsmap puts the risk across pregnancy and childbirth at about 22.6%, roughly 1 in 4[9].
Treatment transforms that number. CDC states the risk of transmitting HIV to a baby “can be 1% or less if the mother takes HIV medicine as prescribed throughout pregnancy, labor, and delivery, and her baby receives HIV medicine for 4–6 weeks after”[4]. aidsmap's estimate with an undetectable viral load is 0.14%, about 1 in 715[9]. This is one of the clearest success stories in HIV medicine, and it depends on early testing and staying in care[4].
Route 4: blood and occupational exposure
Transfusion with HIV-containing blood is the single highest-risk exposure on CDC's list — 9,250 per 10,000[3]. It is also, in the United States, essentially historical: NIH notes that strict blood screening has cut U.S. transfusion risk to less than one in a million[5].
Occupational needlestick risk is 23 per 10,000, about 1 in 420[3][4]. HIV.gov describes workplace HIV transmission as “extremely rare,” and PEP is available after a needlestick[7]. NIH also lists genuinely rare blood routes — a shared razor or toothbrush, pre-chewed food, contact with an open wound — noting these require unusually large amounts of blood plus a direct route in[5].
What does not transmit HIV — and what U=U means
HIV does not survive long outside the body and cannot reproduce outside a human host[2]. CDC states HIV is not transmitted by mosquitoes, ticks, or other insects; through saliva, tears, or sweat; by hugging, shaking hands, sharing toilets, sharing dishes, or closed-mouth social kissing; through sexual activities with no fluid exchange, such as touching; or through the air[2]. WHO adds that HIV “is not spread by kisses, hugs or sharing food,” nor by day-to-day contact or sharing personal objects, food, or water[8]. aidsmap notes HIV is not infectious in saliva, urine, feces, or tears[10]. Deep kissing is only theoretically relevant if there is significant blood exposure, and CDC rates biting, spitting, and throwing body fluids including saliva as negligible risk[3].
And the headline: U=U. NIH states that people who achieve and maintain an undetectable viral load on ART “cannot sexually transmit the virus to others” — established by HPTN 052 and confirmed by the PARTNER and Opposites Attract studies[6].
Related questions
Can mosquitoes spread HIV?
No. CDC states HIV is not transmitted by mosquitoes, ticks, or other insects.
Is kissing safe?
Yes. Closed-mouth or social kissing does not transmit HIV, and HIV is not infectious in saliva. CDC classifies spitting and throwing saliva as negligible risk.
Does “undetectable” mean zero, or just very low?
CDC describes viral suppression as fewer than about 200 copies/mL and undetectable as generally fewer than about 40 copies/mL. For sexual transmission, a durably undetectable viral load means HIV cannot be passed on.
Can someone with HIV have a baby who doesn't have HIV?
Yes — this is now the expected outcome. With HIV medicine taken as prescribed through pregnancy, labor, and delivery, plus 4–6 weeks of medicine for the baby, risk drops to 1% or less.
Related from RiseUpToHIV
References & Sources
- CDC — How HIV Spreads. The fluids that carry HIV, the routes of entry, and perinatal transmission. ↩ ↩ ↩ ↩
- CDC — Ways HIV Is Not Transmitted. Insects, saliva, casual contact, air, and non-fluid sexual activity. ↩ ↩
- CDC — HIV Risk and Prevention Estimates. Per-10,000 estimates for sex, needle sharing, transfusion, and needlestick. ↩ ↩ ↩ ↩ ↩ ↩
- CDC — What Can Increase HIV Risk?. Odds framing, perinatal risk with treatment, and the injection-equipment caveat. ↩ ↩ ↩ ↩ ↩ ↩
- NIH HIVinfo — Understanding How HIV Is Transmitted. Share of diagnoses from sexual contact, transfusion safety, and rare blood routes. ↩ ↩ ↩ ↩ ↩ ↩
- NIH — Undetectable equals untransmittable. Federal U=U statement and the trials behind it. ↩ ↩
- HIV.gov — Post-Exposure Prophylaxis (PEP). PEP after occupational or other exposures; workplace transmission is extremely rare. ↩
- WHO — HIV and AIDS fact sheet. Global guidance that HIV is not spread by kisses, hugs, or sharing food. ↩
- aidsmap — Estimated HIV risk per exposure. “1 in X” odds per act and perinatal risk with and without treatment. ↩ ↩ ↩
- aidsmap — HIV transmission. Why the receptive partner faces higher risk; fluids that are not infectious. ↩ ↩ ↩
Community publications like POZ, Positively Aware, and TheBody inform framing and lived-experience context on RiseUpToHIV. Every clinical, epidemiological, or public-health claim above is anchored to a primary source.