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HIV Infection

HIV Infection

23 May 2023

Causative agent

The infection is caused by Human Immunodeficiency Virus (HIV).

Clinical features

Most HIV infected persons look healthy and have no specific or obvious signs or symptoms during the early phase of infection. HIV attacks and destroys CD4 cells, the disease-fighting cells of the immune system. As the immune system is seriously damaged, the infected person is more vulnerable to certain cancers and a wide range of infections. These types of infections are known as opportunistic infections because they take advantage of a person’s weakened immune system. Without treatment, about half of the infected people will progress to AIDS (Acquired Immunodeficiency Syndrome) in 10 years’ time. AIDS is characterised by development of major clinical complications related to the underlying immunosuppression.

Mode of transmission

HIV is primarily found in the blood, semen, or vaginal fluid of an infected person. The most common ways that HIV is transmitted from one person to another are:

  • by having sex (vaginal, oral, or anal) with an HIV-infected person. In Hong Kong, about 80% of HIV positive people were infected through sexual contact;
  • by sharing needles, syringes or injection equipment with an injecting drug user who is infected with HIV;
  • from HIV-infected women to their babies before or during birth, or through breast-feeding after birth;
  • through receipt of infected blood or blood products.

HIV is not transmitted by day-to-day activities such as shaking hands, hugging, a kiss on the cheek, dining together, or sharing toilet seat. Besides, there is no evidence to show that mosquitoes can transmit HIV.

How is HIV infection diagnosed?

Most people with HIV do not look sick. It is impossible to tell if a person has the virus just by looking at, or talking to, him or her. A person has HIV can pass the virus to others even if he or she is asymptomatic. An HIV antibody test is a test that can tell whether or not a person has been infected by HIV.

Antibodies are usually produced by the body when the body is exposed to an infection. Antibodies are one of the body’s tools for fighting infections. The antibodies against HIV, however, are not protective in nature. HIV antibody testing is a blood test carried out to find out if one has contracted HIV.

If HIV antibodies are present, the test is positive. It means that the person is HIV-infected.

If the result of the test after the window period is negative, it means that the tested person is not infected.

What is Window Period?

Window period is the time between initial infection with HIV and when a positive test result occurs. The window period for HIV testing is 3 months. During the window period, the infected person can transmit the virus even though an HIV antibody test shows a negative result.

When should I have an HIV test?

Everyone who ever had sexual experience is recommended to get tested for HIV. For people who have acquired sexually transmitted infections, early HIV testing should be arranged.

The following populations are known to have a higher risk of contracting HIV. They are recommended to have HIV testing at least every 6 to 12 months:

  • Men who have sex with men
  • Transgenders
  • Sex workers and their clients
  • People who inject drugs
  • People (or their sex partner) who have multiple sex partners
  • Sex partners of HIV positive individuals (until consistent viral suppression was documented)


Although there is no cure yet for HIV infection, significant advances and progresses in the treatment have been made. The antiretroviral agents attack HIV at different stages of its life cycle to inhibit HIV replication, and keep the immune system healthy. People under an HIV treatment can live a healthy and productive life.

An effective antiretroviral therapy can reduce the viral load (the number of virus in human blood) and increase the number of CD4 which helps maintain the immune system, as well as contribute to fewer opportunistic infections and HIV-associated cancers

When people infected with HIV is taking effective antiretroviral therapy and have taken the drugs for around 6 months, or even within 6 months for some people, the viral load can drop to a level that is undetectable (<200 copies/ml) by standard laboratory tests.

Evidence has shown that people with HIV who achieve sustained viral suppression to an undetectable level by antiretroviral drugs have no chance of passing on the virus through sex, i.e. Undetectable = Untransmittable (U=U).

As with other diseases, early detection offers more options for treatment and preventive care.


  1. If you have sex, always protect yourself and your partner by using a condom. Proper and consistent use of a condom for vaginal, oral and anal sex can reduce risk of HIV transmission. Maintaining a mutually monogamous relationship can reduce but not eliminate the risk of HIV infection, because there is no way to tell by looking at someone if he or she has HIV. If your partner refuses to use a condom, opt for alternative sexual activities that do not involve the exchange of body fluids (e.g. touching).
  2. Don't use alcohol or drugs before you have sex. These substances can affect your judgment, making you less likely to protect yourself by using a condom.
  3. Do not share needles, syringes or any other injecting equipment.
  4. If you are pregnant or planning to be, get tested for HIV as soon as possible. Drug treatments are available to help you and reduce the chance of passing HIV to your baby if you are infected.