Hepatitis B is a liver infection caused by the hepatitis B virus (HBV), a DNA virus that specifically targets liver cells. It spreads through contact with infected blood, semen, or other body fluids, and it can range from a short-term illness that resolves on its own to a lifelong chronic infection that damages the liver over time. Globally, it remains one of the most common serious infectious diseases, but it’s also one of the most preventable thanks to a highly effective vaccine.
How the Virus Affects the Liver
HBV has an unusual life cycle. Once it enters a liver cell, it converts its DNA into a stable template that essentially sets up permanent residence inside the cell’s nucleus. The virus then uses an RNA intermediate to copy itself, reverse-transcribing that RNA back into viral DNA. This template DNA is extremely difficult to eliminate, which is a major reason chronic hepatitis B is so hard to cure.
The liver damage itself isn’t caused directly by the virus replicating. Instead, your immune system recognizes infected liver cells and attacks them. In acute infection, a strong immune response can clear the virus but also causes inflammation. In chronic infection, a weaker, ongoing immune response causes slow, progressive damage to liver tissue over years or decades.
How Hepatitis B Spreads
The virus is primarily transmitted when blood, semen, or other body fluids from an infected person enter the body of someone who isn’t protected. Even microscopic amounts of blood can carry enough virus to cause infection. The most common routes include:
- Sexual contact with an infected partner
- Sharing needles or syringes
- Mother to baby during childbirth
- Contact with open wounds or blood from an infected person
- Sharing razors or toothbrushes that may have traces of blood
Although HBV can be found in saliva, it is not spread through kissing, sharing utensils, hugging, or casual contact. You also can’t get it from food, water, or breastfeeding.
Symptoms and Timeline
After exposure, hepatitis B has a long incubation period. It takes an average of 90 days before symptoms appear, though this can range from 60 to 150 days. Some people never develop noticeable symptoms at all, particularly younger adults and children. Adults over 30 are more likely to experience symptoms during the acute phase.
When symptoms do appear, they typically include fatigue, loss of appetite, nausea, vomiting, abdominal pain (especially in the upper right side near the liver), dark urine, clay-colored stools, joint pain, and jaundice (yellowing of the skin and eyes). For some people, acute hepatitis B causes only a mild illness. For others, it can be severe enough to require hospitalization.
Chronic hepatitis B often produces no symptoms for years or even decades. Many people don’t know they’re infected until liver damage has already progressed, which is why screening matters so much.
Acute vs. Chronic Infection
The distinction between acute and chronic hepatitis B is one of the most important things to understand about this disease. Acute infection is the initial phase, lasting up to six months. Most healthy adults who contract HBV as adults clear the virus on their own during this period and develop lasting immunity.
Chronic infection means the virus persists for longer than six months. The risk of becoming chronically infected depends heavily on the age at which a person is exposed. Infants infected at birth have roughly a 90% chance of developing chronic hepatitis B. Children infected between ages 1 and 5 have about a 25 to 50% chance. Healthy adults, by contrast, have less than a 5% risk of the infection becoming chronic. This age-dependent pattern is one reason newborn vaccination is so critical.
Long-Term Complications
Chronic hepatitis B is a serious condition because of what it can do to the liver over time. The ongoing low-grade inflammation gradually replaces healthy liver tissue with scar tissue, a process called fibrosis. As scarring progresses, it can develop into cirrhosis, where the liver becomes so scarred it can no longer function properly. Cirrhosis can lead to liver failure, fluid buildup in the abdomen, internal bleeding, and the need for a liver transplant.
Chronic HBV infection also significantly increases the risk of liver cancer (hepatocellular carcinoma). Unlike most cancers, which require multiple steps of tissue damage before they develop, HBV can contribute to cancer even before cirrhosis sets in. This is why people with chronic hepatitis B are typically monitored with regular liver imaging, even if their liver function tests look normal.
Screening and Diagnosis
The CDC recommends that all adults aged 18 and older get screened for hepatitis B at least once in their lifetime. Anyone who requests testing should receive it regardless of whether they disclose specific risk factors. This universal approach reflects how common undiagnosed infection is.
Screening uses a blood test called a triple panel, which checks three markers. The first, HBsAg, is a protein on the surface of the virus. A positive result means the virus is actively present in your blood, either from a new or chronic infection. The second marker, anti-HBs, is an antibody your body makes after clearing the virus or after vaccination. A positive result means you’re immune. The third, anti-HBc, is an antibody that appears during infection and stays positive for life. It tells your doctor whether you’ve ever been exposed to the virus.
Together, these three markers paint a clear picture. Someone who is immune from vaccination will only test positive for anti-HBs. Someone who recovered from a past infection will be positive for both anti-HBs and anti-HBc. A person with chronic infection will be positive for HBsAg and anti-HBc but negative for anti-HBs. And someone who has never been infected or vaccinated will test negative across the board.
Treatment for Chronic Hepatitis B
Not everyone with chronic hepatitis B needs treatment right away. The decision depends on how much virus is in your blood, whether there’s evidence of liver inflammation, and how much scarring has developed. When treatment is needed, the goal is to suppress the virus to undetectable levels and prevent further liver damage.
The main treatment approach uses oral antiviral medications that block the virus from copying itself. These drugs are taken daily, and between 68 and 90% of people on first-line therapy achieve undetectable virus levels after about 48 weeks. The limitation is that these medications don’t eliminate the stable viral template lodged inside liver cells, so most people need to stay on treatment long-term to keep the virus suppressed. Stopping can lead to a flare of the infection.
A second option, interferon therapy, works by boosting the immune system’s response to the virus. It’s given as injections over a set period rather than indefinitely. The advantage is that it offers a small chance of what’s called a functional cure, where the surface antigen (HBsAg) disappears from the blood and the virus stays suppressed without ongoing medication. This outcome is rare with oral antivirals alone. However, interferon has more side effects and isn’t suitable for everyone.
Vaccination and Prevention
The hepatitis B vaccine is one of the most effective vaccines available. A completed series provides over 90% protection in healthy adults, and that protection lasts at least 30 years, likely longer. It’s given as a series of two or three doses depending on the formulation, spaced over one to six months.
In the United States, vaccination is recommended for all infants at birth, all children and adolescents who weren’t vaccinated earlier, and all adults up to age 59. Adults 60 and older can also receive the vaccine, particularly if they have risk factors. For babies born to mothers with hepatitis B, receiving the vaccine along with a dose of protective antibodies within 12 hours of birth is highly effective at preventing transmission.
Beyond vaccination, practical prevention includes using barrier protection during sex, never sharing needles, and avoiding sharing personal items like razors or toothbrushes that might carry traces of blood. If you think you’ve been exposed and aren’t vaccinated, post-exposure treatment is available and most effective when given within 24 hours.