Since its creation in 1988, the day is commemorated every year on December 1.
Who created the day — and why?
The day was first dreamt up in 1987 by James Bunn and Thomas Netter, two public information officers for the UN Programme on AIDS and was formally recognised by the organisation the following year.
It aimed to raise awareness about the effects of the human immuno-deficiency virus (HIV) and to provide an opportunity for people worldwide to unite in the fight against the disease.
More than 35 million people died from AIDS-related illnesses since 1983, making it one of the most destructive epidemics in history, according to the United Nations.
"Today, we commemorate World AIDS Day—we stand in solidarity with the 78 million people who have become infected with HIV and remember the 35 million who have died from AIDS-related illnesses since the first cases of HIV were reported," said the Executive Director of UNAIDS, Under-Secretary-General of the United Nations, Michel Sidibé.
"The world has committed to end the AIDS epidemic by 2030 as part of the Sustainable Development Goals. We are seeing that countries are getting on the Fast-Track—more than 18 million people are on life-saving HIV treatment and country after country is on track to virtually eliminate HIV transmission from mother to child."
What is HIV and what is AIDS?
HIV stands for human immunodeficiency virus, that breaks down the ability for humans to protect their immune systems.
The most advanced stages of HIV infection is called Acquired Immunodeficiency Syndrome, commonly known as AIDS.
The virus can be transmitted through unprotected sexual intercourse with an infected person, through childbirth, and breastfeeding.
How many people have been infected with the virus?
At the end of last year, there were an estimated 36.7 million people around the world living with HIV. In the same year, 2.1 million people were infected, and 1.1 million died of HIV-related causes.
How far have we gotten with combating HIV or AIDS? Are many people still suffering?
Sub-Saharan Africa is the epicenter of the epidemic. In 2013, the number of people living with HIV in sub-Saharan Africa accounted for 71 percent of the global total.
Have we made progress in combating HIV?
A report released by the Center for Disease Control and Prevention said that HIV diagnoses have declined by nearly 20 percent during the past decade. But progress differs from country to country.
At the same time, AIDS-related deaths have decreased by 45 percent since 2005.
What is the future of AIDS?
There is no cure for HIV and AIDS as yet. But there are many ways of treating the disease that has maximally suppressed the virus. One such treatment is antiretroviral therapy (ART) which helps stop the progression of the disease.
South Africa launched a major clinical trial of an experimental vaccine against the AIDS virus on Wednesday. The trial, which will last for four years, will involve more than 5,400 men and women ages 18 to 35 in a number of areas around South Africa.
It is one of the biggest clinical trials involving the disease ever undertaken and has revived hopes of a breakthrough in the battle against AIDS.
"A highly efficient vaccine would be a game-changer but the results of these trials will take years," said the head of the HIV virology section at South Africa's National Institute for Communicable Diseases (NICD), Lynn Morris.