Hepatitis B spreads through contact with infected blood, semen, or other body fluids. The virus is highly infectious, transmitting far more efficiently than HIV through the same general routes. It can survive on surfaces for at least seven days, meaning even dried blood you can’t see may carry a risk.
Sexual Contact
Unprotected sex with someone who has hepatitis B is one of the most common ways adults get infected. The virus is present in both semen and vaginal fluids, so vaginal, anal, and oral sex all carry risk. You don’t need to have visible sores or bleeding for transmission to occur. Mucosal membranes (the soft, moist tissue lining the genitals, rectum, and mouth) can absorb the virus directly.
Blood-to-Blood Contact
Sharing needles, syringes, or any equipment used to inject drugs is an efficient route of transmission. Even a tiny amount of blood left on a needle is enough. For context, an unvaccinated person who gets stuck with a contaminated needle has a 6 to 30 percent chance of becoming infected from that single exposure, which is dramatically higher than the risk for HIV from the same type of injury.
This blood-to-blood principle extends beyond drug use. Tattoos and body piercings done with improperly sterilized equipment can transmit hepatitis B if the tools carry traces of infected blood from a previous client. The same applies to unregulated dental or medical procedures, particularly in parts of the world where sterilization standards are inconsistent.
Mother to Child During Birth
A pregnant person with hepatitis B can pass the virus to their baby during delivery. The risk depends on how much virus is circulating in the mother’s blood. Mothers with high viral loads face a 70 to 90 percent chance of transmitting the infection to their newborn. Even mothers with lower viral activity still carry a 12 to 25 percent risk.
This matters enormously because of what happens next. Infants who get infected at birth have up to a 90 percent chance of developing a chronic, lifelong infection. That’s why hospitals routinely test pregnant people for hepatitis B and give newborns a vaccine and protective antibody shot within hours of birth. When that post-birth treatment is given, transmission drops to nearly zero.
Breastfeeding, notably, does not appear to increase risk. A study comparing breastfed and non-breastfed babies born to infected mothers found no meaningful difference in transmission rates, and the risk effectively disappeared when babies received the standard post-birth prevention.
Sharing Personal Items
Because hepatitis B survives on surfaces for at least a week, everyday items that might carry small amounts of blood can be a transmission route. Razors and toothbrushes are the main culprits. A nick from a shared razor or bleeding gums on a shared toothbrush can leave enough virus behind to infect the next person who uses it. Nail clippers and glucose monitors fall into the same category.
This is particularly relevant for people living with someone who has hepatitis B. The risk isn’t from casual contact, but from these specific shared items that can carry microscopic traces of blood.
How It Does Not Spread
Hepatitis B does not spread through food, water, or casual contact. Shaking hands, hugging, sitting next to someone, sharing meals, using the same toilet, or being coughed or sneezed on will not transmit the virus. You cannot get it from mosquito bites. Kissing carries only a theoretical risk unless both people have open sores or bleeding gums.
Why Age at Infection Matters
The younger you are when you catch hepatitis B, the more likely it is to become a chronic infection rather than one your body clears on its own. The numbers are striking. Infants infected at birth face up to a 90 percent chance of chronic infection. Children under five have roughly a 20 to 60 percent chance. For older children and adults, the risk drops to 5 to 10 percent.
A study from Senegal illustrated how sharply this risk shifts even within early childhood. Among children infected before six months of age, 82 percent developed chronic hepatitis B. For those infected between six months and one year, the rate was 54 percent. After age four, it fell to about 6 percent. This age gradient is the primary reason childhood vaccination programs exist: preventing infection in the years when it’s most likely to become permanent.
Adults who get hepatitis B typically clear the virus within a few months, often without realizing they were infected. But that 5 to 10 percent who don’t clear it face long-term liver damage, so the infection is worth preventing at any age.
How Soon It Shows Up
After exposure, the virus takes time to become detectable. The earliest blood marker, called HBsAg, typically appears about four weeks after infection, though it can show up anywhere from one to nine weeks later. This means a test taken too soon after a potential exposure could come back negative even if you’re infected. If you think you’ve been exposed, testing at the right time is important for an accurate result.
Symptoms, when they occur at all, usually appear two to three months after exposure. Many adults never develop noticeable symptoms, which is one reason the virus spreads so effectively. People can carry and transmit hepatitis B without knowing they have it.