If you are sexually active, use a needle to inject yourself with drugs, or share a needle with someone who injects drugs, you can be infected with HIV.  We encourage you to read this story and to consult with your doctor to learn more about HIV/AIDS. As a Consumer Reports Consumer Reports Health subscriber, you’ll have access to our expert research and recommendations, and you’ll be more confident and knowledgeable about which treatments are best for you.

• HIV lives in blood and in men's semen and women's vaginal fluid. You can get the virus when fluids from an infected person get inside your body.
• Most people get HIV by having unprotected sex with someone who has the virus.
• You can also get the virus by sharing a needle or a syringe with someone who injects drugs.
• Pregnant women with HIV can pass the virus to their baby during pregnancy, birth, or when breast-feeding, but there are treatments to prevent that from happening.
• You can't get HIV from kissing on the cheek or sharing a fork or bar of soap with someone who has the virus.
• You can have HIV for many years before it starts to affect your health, when it's called AIDS.
• The only way to find out whether you have HIV is to have a blood test.
• In the United States, it is thought that more than 1 million people have HIV. Worldwide more than 38 million people are infected with HIV.

HIV is an acronym for human immunodeficiency virus. If you become infected with HIV, you slowly lose the ability to defend yourself against other infections and certain types of cancer. HIV destroys cells that are part of your immune system, the name given to the parts of your body that protect you from infections. This reduces your ability to fight off certain other infections. When this happens, you have AIDS. AIDS is an acronym for acquired immunodeficiency syndrome.

Your immune system is the name given to the parts of your body that protect you against infections. Bacteria, viruses, funguses and parasites are all types of germs that may make you sick. If you get infected, your immune system tries to kill the bug.

Your immune system includes many different types of immune cells. They travel in your blood looking for infections. When your immune cells meet a virus, they recognize that the virus may be dangerous. They work together to try to destroy the virus. Your immune system also kills any of your own cells that have begun to grow incorrectly. For example, it can help stop cancers from developing.

Certain cells called CD4 cells (or T helper cells) help pass a message to other cells to help destroy the virus. It can take many days for your immune system to get rid of a virus.

HIV doesn't spread through the air like cold and flu viruses. So you can't catch it by being next to someone who has HIV and breathing in after they have sneezed.

HIV lives in people's blood and in some other body fluids. To get HIV, one of these fluids from someone with HIV has to get into your blood. The chance of catching HIV is different with different fluids and with different activities. For example, it is easier to become infected with HIV from anal sex than through vaginal sex.

Blood contains the most virus particles. You only need a tiny amount of blood from someone with HIV to get into your body to become infected.

In men, HIV also lives in semen. And it may be possible to get HIV from ejaculate, the fluid ("pre-come") that comes out of the penis before ejaculation. In women, HIV lives in vaginal fluid. In women who have recently had a baby, the virus can get into breast milk.

It's important to remember that you can't get HIV from saliva, tears, sweat, feces or urine.

HIV can get into your blood through cuts or sores on your skin. HIV can get through the thin, moist lining of your anus, rectum or genitals. These thin layers are called mucous membranes. HIV can also enter your body through the lining of your mouth and eyes.

HIV can't get through normal skin that's unbroken. So you won't get infected if some blood from a person with HIV splashes on your unbroken skin.

Here are the main ways that people get infected with HIV:

• Having unprotected sex (this is the most common way people get HIV)
• Sharing needles or syringes
• Infection passed from mother to baby
• Accidentally sticking yourself with an infected needle or being splashed with blood on broken skin in a laboratory accident (this is very rare)
• Receiving infected blood in a transfusion (this is also rare). 

There are lots of myths about how you can get HIV. For example, some people worry they can get it by kissing.  They can’t.

You're at risk of getting HIV if:

• You have unprotected sex with someone who is infected. Unprotected means not using a condom.
• You share someone else's needle or syringe to inject drugs. If you inject yourself with any kind of drug, you should never use needles or syringes that someone else has used.
• Your mother had the virus when you were born.
• You're a health-care worker (because you may have an accident with a needle).
• You have a blood transfusion.

HIV is a type of virus called a retrovirus. When it meets your immune cells it enters and destroys them, so your immune system can't fight back effectively and get rid of HIV, like it does with other viruses.

When HIV gets into your body, the virus gets inside your CD4 cells, where it can live for many years. At some point, HIV starts to make thousands of copies of itself. These copies of HIV leave the CD4 cell, killing it at the same time. These copies of HIV move on to other CD4 cells, eventually killing them.

Gradually, the number of CD4 cells in your blood (called your CD4 count) drops very low. With fewer CD4 cells in your blood, you start to get infections and types of cancer that you wouldn't normally get. When this happens, doctors say you have AIDS.

It would be devastating to find out that you have AIDS. But having HIV or AIDS isn't as bad news as it used to be. Treatments can't get rid of HIV completely, but they can keep HIV and other infections under control. Read more about how to treat HIV and AIDs in the "What treatments work" section of our complete condition report.

Even without treatment you may not get other infections for 10 years after you were infected with HIV, and that's without treatment to slow down the virus. If you take treatments, it may be 20 years or more before you start getting additional infections. But it's too soon to say how well drug treatments for HIV work because the new combinations have not been used for very long.

Most people get some mild symptoms about two weeks to four weeks after they get infected with HIV. More serious symptoms usually don't happen for around 10 years.

You can easily miss the early symptoms because they're like the symptoms you get with other virus infections, such as the flu. Once the early symptoms have cleared up, most people don't have any other signs of infections for many years. So it's hard to tell whether you have HIV.

It takes a long time for HIV to damage your immune system enough to make you sick. Most people show some signs of a weak immune system after about 10 years to 12 years. Doctors call these later symptoms. But if you get treatment early, it may be much longer before you experience these sorts of symptoms.

Most people who get infected with HIV get some symptoms after a few weeks. They are like the symptoms you get with a cold or the flu. Doctors call these early symptoms a seroconversion illness, which means that your body is starting to make antibodies against HIV. Antibodies are small particles that your immune system makes to help fight infections.

The symptoms of this early stage usually last about three weeks and clear up without any treatment.

You may get:

• Fever
• Rash
• Headaches
• Sore throat
• Loss of appetite
• Aching muscles and joints.

You might also have swollen lymph nodes (the glands in your armpits, groin and neck) or have recurrent cold sores.

Because some of these symptoms are so general many people don't realize that they have been infected with HIV. But if you think that you may have caught HIV, you should see a doctor specializing in genitourinary medicine. This is important because:

• If you do get a positive HIV test, you will get support and advice, including advice about when to start treatment.
• At this early stage you're very infectious, which means it's very easy for you to spread the virus to other people. If you know you have the virus you can avoid spreading it.

About 10 years to 12 years after getting infected with HIV many people who are not taking drug treatment start to get opportunistic infections, infections you get when your immune system isn't working well. Examples of opportunistic infections are pneumonia and tuberculosis. When you're healthy you don't usually get these infections because your immune system fights them off.

To be diagnosed with AIDS you have to have one opportunistic infection out of a specific list or a CD4 count below 200. You may also get certain types of cancer. Opportunistic infections and cancer can cause many different symptoms. You may have an opportunistic infection if you get any of the following:

• A bad cough
• Shortness of breath
• Difficult or painful swallowing
• Confusion or forgetfulness
• Diarrhea
• Fever
• Problems seeing clearly
• Nausea, stomach cramps and vomiting
• Weight loss
• Extreme tiredness
• Bad headaches
• Loss of consciousness (coma).

These symptoms show that the immune system is damaged and can't protect you well against infections. You may need treatment once you get these infections, and you can also receive treatment to reduce your chances of getting these infections.

The only way to find out whether you have HIV is to have a blood test. You can't tell if someone has HIV by looking at them. Some people don't get any symptoms for 10 years after becoming infected with HIV. For those 10 years, they look and feel well.

It's important to get a blood test if you think you may have been infected. There are several different kinds of blood tests.

Most people worry a lot about having an HIV test. It's a big decision. But if you're worried that you might have been in contact with the virus then there are a few reasons why you should have a test sooner rather than later.

• If you have a positive result you will also get information about the damage HIV has caused to your immune system and to help you decide when to start treatment.
• You'll be able to take care to avoid spreading the virus to other people.
• If you're pregnant, or planning to become pregnant, there are treatments that can reduce the chances of your baby getting the virus.

If you get a positive HIV blood test, it means that you have been infected with HIV. And it means you can pass it on to other people. It doesn't mean that you have AIDS.

Once you're infected with HIV, the virus starts to attack your immune system, the cells that protect you from infections. But it takes many years for this damage to affect your health. And there are drugs that can help you stay healthy for a long time.

Your doctor will usually do two more blood tests to see whether HIV has damaged your immune system. These tests are called a viral load test and a CD4 count test.

Getting a positive test result can be a huge shock. And you'll probably have lots of questions. There are many people and organizations you can turn to for advice.

Here's what we know about the number of people in the United States with HIV.

• Between 1 million and 1.85 million people are living with either HIV or AIDS. But about a quarter of them don't know they have it.
• HIV was discovered in 1983. But infection of the first person in the United States probably happened in the late 1970s.
• About 39 million people around the world are infected with HIV. And more than 20 million people have died from it. Most people with HIV live in developing countries.
• About 40,000 Americans become infected with HIV each year. This number has stayed about the same for the past 10 years.
• About 7 in 10 new cases are in men.
• About half a million Americans have died from AIDS. AIDS is the fifth leading cause of death in the United States among people ages 25 to 44.

Having HIV isn't as bad news as it used to be. Treatments can help you stay healthy for many years.

Without treatments that slow down HIV, you may start to get ill about 10 years after being infected. With treatments it may take 20 years or more to get ill, but effective treatments have not been in use for long enough to be sure. Some experts believe that people who have good treatment and who take care of themselves may be able to live a normal lifespan. But everyone is different, so we can't say for sure what will happen to you.

In the United States, the most common treatment is a combination of three or four antiretroviral drugs (combination therapy). These drugs slow down the damage HIV does to your immune system. Since this kind of therapy was first used, the number of Americans dying from AIDS has fallen by more than two-thirds.

But if you've been told that you have HIV, you probably feel scared. The facts about what happens when you are first diagnosed, what happens in the first few years after diagnosis, what happens if you have advanced HIV infection, what you can do to help yourself and stay healthy, what treatments work, and Treatment Ratings are all available to subscribers to ConsumerReportsHealth.org. To help you understand what can happen see our in-depth, complete condition report on HIV infection.