Causes
HIV is caused by a virus. It can be transmitted through sexual contact, illicit drug use, or sharing needles, as well as through contact with infected blood. It can also be transmitted from mother to child during pregnancy, childbirth, or breastfeeding.

HIV destroys white blood cells called CD4 T cells. These cells play a key role in the body's fight against disease. The fewer CD4 T cells you have, the weaker your immune system.

When does HIV become AIDS?
You can have an HIV infection with few or no symptoms for years before it develops into AIDS. AIDS is diagnosed when the CD4 T cell count drops below 200 or when a complication occurs that only occurs with AIDS, such as a serious infection or cancer.

How HIV is transmitted
You can become infected with HIV if infected blood, semen, or vaginal fluids enter your body. This can happen in the following cases:

Having sexual intercourse. You can become infected if you have anal or vaginal sex with an infected partner. Oral sex poses less risk. The virus can enter the body through sores in the mouth or small tears that sometimes occur in the rectum or vagina during sex.
Sharing needles to inject illicit drugs. Sharing infected needles and syringes increases the risk of contracting HIV and other infectious diseases, such as hepatitis.
Receiving a blood transfusion. Sometimes, the virus can be transmitted through a donor's blood. Hospitals and blood banks test the blood supply for HIV. Therefore, this risk is low where these precautions are taken. In low-resource countries, which do not have the capacity to test all donated blood, the risk may be higher.
Being pregnant, giving birth, or breastfeeding. Pregnant people living with HIV can transmit the virus to their babies. HIV-infected people who receive treatment for the infection during pregnancy can significantly reduce the risk to their babies.

How HIV is not transmitted
You cannot become infected with HIV through casual contact. This means you cannot get HIV or get AIDS by hugging, kissing, dancing, or shaking hands with someone who is infected.

HIV is not spread through air, water, or insect bites. You cannot get HIV by donating blood.

Risk factors
Anyone of any age, race, sex, or sexual orientation can have HIV or AIDS. However, the risk of contracting HIV or AIDS is higher in the following circumstances:

Having unprotected sex. Use a new latex or polyurethane condom every time you have sex. Anal sex is riskier than vaginal sex. The risk of contracting HIV increases if you have more than one sexual partner.
You have a sexually transmitted infection. Many sexually transmitted infections cause open sores on the genitals. These sores allow HIV to enter the body.
Injecting illicit drugs. If you share needles and syringes, you can be exposed to infected blood.

Complications
HIV infection weakens the immune system. This infection makes you more likely to get many infections and certain types of cancer.

Common HIV/AIDS Infections
Pneumocystis carinii pneumonia. This fungal infection can cause severe illness. In the US, it is rare due to HIV and AIDS treatments. However, this infection is still the most common cause of pneumonia in people with HIV.
Candidiasis. Candidiasis is a common HIV-related infection. It produces a thick white coating on the mouth, tongue, esophagus, or vagina.
Tuberculosis. Tuberculosis is a common opportunistic infection linked to HIV. It is a leading cause of death worldwide in people with AIDS. In the US, it is rare thanks to the widespread use of HIV medications.
Cytomegalovirus. This herpes virus is common and is transmitted through body fluids, such as saliva, blood, urine, semen, and breast milk. A healthy immune system inactivates the virus, but it remains in the body. If the immune system weakens, the virus becomes active again and causes damage to the eyes, digestive system, lungs, or other organs. Cryptococcal meningitis. Meningitis is the swelling and irritation or inflammation of the membranes and fluid surrounding the brain and spinal cord, also known as the meninges. Cryptococcal meningitis is a common HIV-related infection of the central nervous system. It is caused by a fungus found in the soil.
Toxoplasmosis: Toxoplasma gondii, a parasite primarily spread by cats, causes this infection. Infected cats shed the parasites in their feces. These parasites then spread to other animals and humans.

Toxoplasmosis can cause heart disease. When it spreads to the brain, it can cause seizures. It can also be fatal.

Common Cancers in People with HIV/AIDS
Lymphoma. This cancer begins in the white blood cells. One of the most common early signs is painless swelling of the lymph nodes, usually those in the neck, armpits, and groin.
Kaposi's Sarcoma. This is a tumor in the walls of blood vessels. Kaposi's sarcoma usually presents as pink, red, or purple sores or lesions on the skin and in the mouth of people with fair skin. In people with black or dark skin, the lesions may appear dark brown or black. Kaposi's sarcoma can also affect internal organs, including the lungs and organs of the digestive system.
Human Papillomavirus-Related Cancers. These are cancers caused by infection with the human papillomavirus. These include anal, oral, and cervical cancer.

Other complications
Wasting syndrome. If HIV or AIDS is left untreated, it can cause significant weight loss. This weight loss is often accompanied by diarrhea, weakness, and fever.
Brain and nervous system or neurological complications. HIV can cause neurological symptoms such as confusion, poor memory, depression, anxiety, and difficulty walking. Neurological conditions associated with HIV can range from mild symptoms, such as behavioral changes and reduced mental functioning, to severe dementia causing weakness and an inability to function.
Kidney disease. HIV-associated nephropathy is the swelling, irritation, or inflammation of the small filters in the kidneys. These filters remove excess fluid and waste from the blood and send it into the urine. Kidney disease most commonly affects Black and Hispanic people.
Liver disease. Liver disease is also a major complication, especially in people who also have hepatitis B or hepatitis C.

Prevention
There is no vaccine to prevent HIV infection and no cure for HIV or AIDS, but you can protect yourself and others from infection.

To help prevent the spread of HIV:

Consider PrEP. There are two PrEP medications that are taken orally and one that is given as an injection. The oral medications are emtricitabine with tenofovir disoproxil fumarate (Truvada) and emtricitabine with tenofovir alafenamide fumarate (Descovy). The injectable medication is cabotegravir (Apretude). PrEP can reduce the risk of sexually transmitted HIV infection in people at very high risk.

PrEP can reduce the risk of acquiring HIV through sexual intercourse by more than 99% and through injection drug use by more than 74%, according to the Centers for Disease Control and Prevention. The effects of Descovy have not been studied in people who engage in receptive vaginal intercourse, which is penetrative sex.

Cabotegvir (Apretude) is the first PrEP approved by the U.S. Food and Drug Administration that can be administered as an injection to reduce the risk of sexually transmitted HIV infection in people at very high risk. The injection is administered by a healthcare professional. After two monthly injections, Apretude is administered every two months. The injection is an option instead of a daily PrEP pill.

Your healthcare professional prescribes these medications to prevent HIV only for people who are not yet infected. You need an HIV test before starting PrEP. You must be tested every three months for pills or before each injection while taking PrEP.

You must take the pills every day or carefully follow the injection schedule. To protect yourself from other sexually transmitted infections, you must practice safe sex. If you have hepatitis B, you should consult an infectious disease or liver specialist before starting PrEP therapy.

Use treatment as prevention. If you have HIV, taking HIV medications can prevent your partner from becoming infected with the virus. If your blood tests do not show any virus, it means your viral load cannot be detected and you will not transmit the virus to anyone through sex.

If you use treatment as prevention, you must take the medications exactly as prescribed and undergo regular checkups.

Use PrEP if you have been exposed to HIV. If you think you were exposed through sex, needles, or at work, contact your healthcare professional or go to the emergency room. Using post-exposure prophylaxis as soon as possible within the first 72 hours can greatly reduce your risk of contracting HIV. You must take the medication for 28 days.
Use a new condom every time you have anal or vaginal sex. There are male and female condoms. If you use lubricant, make sure it's water-based. Oil-based lubricants can deteriorate condoms and cause them to break.

During oral sex, use a cut condom or a latex drape, which is a piece of medical-grade latex, without lubricant.

Inform your sexual partners that you have HIV. It's important to tell all your current and past sexual partners that you are infected with HIV so they can request testing.
Use clean needles. If you use needles to inject illicit drugs, make sure they are sterile. Don't share them. Use needle exchange programs in your community. Seek help for your illicit drug use.
If you are pregnant, seek medical attention immediately. You can transmit HIV to your baby. However, if you receive treatment during pregnancy, you can greatly reduce the risk to your baby.
Consider male circumcision. Studies show that circumcision, which is the removal of the foreskin of the penis, can help reduce the risk of HIV infection.