What Is Hepatitis B: Virus, Acute vs Chronic, and Testing
Content last reviewed 2026-09-12 · Educational information — not medical advice
Quick answer
Hepatitis B is a liver infection caused by the hepatitis B virus (HBV), spread through blood, sex, and birth. It can be acute (short-lived) or chronic (lifelong), and chronic infection is far more likely when acquired in infancy. A surface antigen test (HBsAg) shows active infection.
What is hepatitis B?
Hepatitis B is an infection of the liver caused by the hepatitis B virus (HBV). The virus attacks liver cells, and when symptoms appear they tend to be the classic hepatitis trio: fatigue, dark urine, and jaundice, sometimes with nausea or pain in the upper right belly. Many people feel nothing at all, which is why the infection often goes unnoticed for years. HBV spreads through blood, sex, and from mother to baby at birth — not through casual contact, coughing, or shared meals. Because it can be silent, testing is the only reliable way to know your status.
What's the difference between acute and chronic hepatitis B?
Acute hepatitis B is the first six months after infection. Most adults fight the virus off and clear it completely, developing protective antibodies along the way. Chronic hepatitis B means the virus stays in the body long-term, which raises the risk of liver damage, cirrhosis, and liver cancer over many years. The odds flip dramatically with age: about 90% of infected infants develop chronic infection, while only 2-6% of adults do. That's why babies are vaccinated at birth and why early testing matters so much.
How is hepatitis B transmitted?
HBV lives in blood and other body fluids. The main routes are unprotected sex, sharing needles or drug equipment, and birth from an infected mother. It can also spread through needlesticks, shared razors or toothbrushes, and tattoo or piercing equipment that isn't properly sterilized. You cannot catch it from hugging, handshakes, toilet seats, food, water, or casual contact. Understanding the real routes helps you protect yourself without unnecessary worry about everyday interactions.
How is hepatitis B tested?
The core marker is the hepatitis B surface antigen (HBsAg) — a positive result means an active infection, acute or chronic. The surface antibody (anti-HBs) shows immunity from vaccination or past infection, and the core antibody (anti-HBc) shows past or ongoing exposure. After exposure, HBsAg can be detected in about 3-6 weeks, so testing too early can miss an infection. A hepatitis panel combines these markers to give a complete picture of your status.
What are the symptoms of hepatitis B?
When symptoms appear, they usually show up 1-4 months after exposure and can include fatigue, dark urine, clay-colored stool, jaundice (yellowing of skin and eyes), nausea, vomiting, and pain in the upper right abdomen. But a large share of people, especially children, have no symptoms at all. Because silent infection is common, many people only learn their status through routine blood work. If you have risk factors, don't wait for symptoms — testing is the only reliable way to know.
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Frequently asked questions
Can hepatitis B be cured?
Acute hepatitis B clears on its own in most adults. Chronic hepatitis B is managed with antiviral medication rather than cured outright, and treatment protects the liver over the long term.
Is hepatitis B an STD?
Yes — hepatitis B is sexually transmitted and is one of the most common viral STIs worldwide. Condoms reduce risk and the vaccine offers the strongest protection.