Simian Immunodeficiency Virus (SIV) is a virus that infects primates, particularly monkeys and apes. It is similar to the human immunodeficiency virus (HIV), which causes AIDS in humans. SIV can weaken the immune system of infected animals, making them more susceptible to other infections and diseases. SIV is primarily transmitted through bodily fluids, such as blood and saliva. While it does not typically infect humans, some strains of SIV are believed to have crossed over to humans, leading to the emergence of HIV. Understanding SIV helps researchers study the origins and evolution of HIV and develop potential treatments.