World AIDS Day is held each year on December 1 as an opportunity to raise awareness about the fight against HIV, show support for people living with HIV and to commemorate people who have died from AIDS.

Here are facts you should know about HIV and AIDS:

. AIDS stands for Acquired Immune Deficiency Syndrome: Acquired means you can get infected with it; Immune Deficiency means a weakness in the body’s system that fights diseases; Syndrome means a group of health problems that make up a disease.

AIDS refers to the most advanced stages of HIV infection (human immunodeficiency virus), defined by the occurrence of any of more than 20 opportunistic infections or related cancers. Being HIV-positive, or having HIV disease, is not the same as having AIDS. Many people are HIV-positive but don't get sick for many years.

. HIV can be transmitted through: unprotected sexual intercourse or oral sex with an infected person; transfusions of contaminated blood; the sharing of contaminated needles, syringes or other sharp instruments; or the transmission between a mother and her baby during pregnancy, childbirth and breastfeeding.

. Globally, an estimated 36.7 million people were living with HIV in 2015, and 1.8 million of these were children (i.e., under the age of 15). The vast majority of people living with HIV are in low- and middle-income countries. An estimated 2.1 million people were newly infected with the virus in 2015. An estimated 1.1 million people died from AIDS-related causes in 2015, down from two million in 2000.

. An estimated 150,000 children became infected with HIV in 2015, down from 290,000 in 2010—a decline of nearly 50 percent.  Most of these children live in sub-Saharan Africa and were infected by their HIV-positive mothers during pregnancy, childbirth or breastfeeding. Mother-to-child-transmission of HIV is almost entirely avoidable, though access to preventive interventions remains limited in many low- and middle-income countries. In 2015, Cuba was the first country declared by the World Health Organization as having eliminated mother-to-child transmission of HIV. In June 2016, Armenia, Belarus, and Thailand were also validated for eliminating mother-to-child HIV.

. HIV is the result of multiple cross-species transmissions of simian immunodeficiency viruses (SIVs) naturally infecting African chimpanzees. While no one knows how the cross-species transmission occurred, the origin of the AIDS pandemic has been traced to the 1920s in the city of Kinshasa, in what is now the Democratic Republic of Congo. An international team of scientists say a “perfect storm” of a roaring sex trade, rapid population growth, and unsterilized needles used in health clinics probably spread the virus. HIV spread in the Congo for almost sixty years before it recognized as a new disease in 1981.