Different viruses, called by the letters A through E, cause what is collectively known as hepatitis. Some are much more dangerous than others.
This week - all about five diseases caused by five different viruses. These diseases all attack the liver. Doctors have one name for them: hepatitis.
The liver is in the upper right part of the stomach area. This dark red organ is big - it weighs more than one kilogram. And it has a big job.
The liver helps to clean the blood and fight infection. It also helps break down food and store energy until the body needs it.
Hepatitis destroys liver cells. But some kinds of hepatitis are much more serious than others.
The five kinds that scientists have identified over the years are called by the letters A through E. Which kind a person has can only be identified by tests for antibodies in the blood.
Antibodies are special proteins that the defense system produces in reaction to a threat. Identify the antibody and you identify the threat.
Hepatitis A is usually spread through human waste in water or food. It is in the same group of viruses as those that cause polio.
The hepatitis A virus causes high body temperature, tiredness and pain. It causes problems with the stomach and intestines, making it difficult to eat or digest food.
Also, the skin of a person with hepatitis may become yellow. This is a sign that the liver is not operating normally.
To help prevent the spread of hepatitis A, people should wash their hands after they use the bathroom or change a baby's diaper. People should also wash their hands before they eat or prepare food.
Hepatitis A can spread quickly to hundreds or thousands of people. But this virus is deadly in less than one percent of cases.
In fact, many people never even get sick. Those who do generally recover within two months.
The Centers for Disease Control Web site - cdc.gov - says the usual treatment is bed rest and a balanced diet. Also, a person should avoid alcohol for at least six months.
People who have had hepatitis A cannot get it again. And there is a vaccine that can prevent hepatitis A.
The World Health Organization calls Hepatitis B "one of the major diseases of mankind." It says two thousand million people, or one-third of all people, are infected with the hepatitis B virus.
The highest rates are in developing countries.
This virus is in the same group as the herpes and smallpox viruses. There have been hepatitis B vaccines since the early nineteen eighties.
Hepatitis B spreads when blood from an infected person enters the body of another person.
An infected mother can infect her baby. The virus can also spread through sex, and if people share injection devices. Blood products from an infected person can spread hepatitis B.
And people can get infected if they share personal-care items that might have blood on them, like shaving razors or toothbrushes.
Worldwide, most hepatitis B infections happen in children. Young children are the ones most likely to develop a lifelong infection.
The W.H.O. estimates that more than three hundred fifty million people are chronically infected with the virus.
The risk of a lifelong infection is small for those infected after age four.
But about ninety percent of those infected with hepatitis B during their first year develop chronic infections.
They are at high risk of death from liver disease and liver cancer. When scientists developed a hepatitis B vaccine, it was considered the first medicine to protect people against cancer.
Hepatitis C is even more dangerous. Like hepatitis B, it spreads when blood from an infected person enters someone who is not infected.
It belongs to the same group of viruses as yellow fever and West Nile virus.
Most people infected with hepatitis C develop chronic infections, often without any signs. They are at high risk for liver disease and liver cancer.
The World Health Organization says about one hundred eighty million people are infected with hepatitis C. And each year as many as four million more become infected.
The W.H.O. says the highest rates are in Africa, Latin America and Asia.
Scientists have been working to develop a vaccine against hepatitis C. The virus was first observed in nineteen seventy-four.
But it was not officially recognized as a new kind of hepatitis until nineteen eighty-nine.
Research suggests that each year as many as twelve thousand people in the United States die of hepatitis C.
A study by the National Institutes of Health found high rates of the virus in some groups, including prisoners and homeless people.
People who received blood and blood products before nineteen ninety-two also have an increased risk.
Hepatitis D is also spread through blood but only infects people who already have hepatitis B. The virus greatly increases the chance of severe liver damage.
The W.H.O says ten million people around the world are infected with hepatitis D.
Doctors can treat some cases of hepatitis B, C and D. The drugs used are very costly, however. But they are less costly than another treatment possibility: getting a new liver.
The fifth virus is hepatitis E. Experts say it spreads the same way as hepatitis A -through infectious waste.
Outbreaks often result from polluted supplies of drinking water. Medical science recognized hepatitis E as a separate disease in nineteen eighty.
Hepatitis E is also found in animal waste. Studies have shown that the virus can infect many kinds of animals, including pigs, cows and monkeys.
The W.H.O. says epidemics of hepatitis E have been reported in Central and Southeast Asia, North and West Africa and Mexico.
There are no vaccines or medicines for hepatitis E. Most people recover, usually in several weeks or months. But the disease can cause liver damage. And, in some cases, hepatitis E is deadly.
The virus is especially dangerous to pregnant women. Twenty percent of women with hepatitis E in the last three months of pregnancy die.
There are no cures for any kind of hepatitis. The only way to protect against infection is to avoid contact with the virus.
And that may be difficult or impossible.
But experts say vaccines can greatly reduce the risk of hepatitis A and B. And there are other steps people can take to protect themselves.
As we said, a person can get some kinds of hepatitis through sex or sharing needles.
Supplies of blood products should be carefully tested for hepatitis. People in high-risk groups and those who have had hepatitis should not give blood.
Exceptions may be made for people who had hepatitis A before age eleven. Donated organs can also spread hepatitis.
Experts say the only way to control the spread of hepatitis is through preventive measures. Even something as simple as washing your hands.