SIV, or Simian Immunodeficiency Virus, is a virus that primarily infects non-human primates, such as monkeys and apes. It is similar to the Human Immunodeficiency Virus (HIV) that affects humans. SIV can lead to a disease similar to AIDS in these animals, but many species have developed a level of tolerance to the virus, allowing them to live relatively healthy lives despite the infection. Research on SIV is important for understanding the origins of HIV and the mechanisms of viral transmission. Studying SIV in its natural hosts helps scientists learn about immune responses and potential treatments for HIV in humans.