Short answers, pulled from the story.
HIV is one of two species of Lentivirus, a subgroup of retrovirus, that infect humans. It infects vital immune cells such as CD4+ T cells, macrophages, and dendritic cells, driving CD4+ T cell numbers down. When those cells fall below a critical level, cell-mediated immunity is lost and the body becomes open to the opportunistic infections and cancers that define AIDS.
HIV-1 is more virulent and more infective than HIV-2 and causes the majority of HIV infections globally, while HIV-2 transmits poorly and is largely confined to West Africa. HIV-1 originated from SIVcpz in wild chimpanzees, and HIV-2 from SIVsmm in the sooty mangabey. In 2020 they were named Lentivirus humimdef1 and Lentivirus humimdef2.
HIV is most often a sexually transmitted infection, passed through contact with blood, pre-ejaculate, semen, and vaginal fluids. Non-sexual transmission can occur from an infected mother to her infant during pregnancy, during childbirth through blood or vaginal fluid, and through breast milk.
U=U stands for Undetectable equals Untransmittable, meaning that HIV is not transmitted through sex when the HIV-positive person maintains a consistently undetectable viral load below 50 copies per millilitre through antiretroviral treatment. The concept was first proposed by the Swiss Federal Commission for AIDS/HIV in 2008. Across four studies, 4,097 couples reported 151,880 acts of condomless sex with zero phylogenetically-linked transmissions when the positive partner was undetectable.
HIV has the highest known mutation rate of any microbe, with estimates as high as 4.1 times 10 to the minus 3 substitutions per base pair. This comes from its fast replication cycle of about ten billion virions per day, the error-prone copying by reverse transcriptase, and recombination between the two RNA genomes carried in each particle.
AIDS was first clinically observed in 1981 in the United States, and the first news story appeared on the 18th of May 1981, in the gay newspaper New York Native. In 1983 two groups, led by Robert Gallo and by Francoise Barre-Sinoussi and Luc Montagnier, independently reported a novel retrovirus in the journal Science. The earliest well-documented human case dates to 1959 in the Belgian Congo.