I have been led to believe the following definitions:
Stands for Sexually Transmitted Infection — an infection caused by bacteria or virus which has been transmitted through sexual contact.
Stands for Sexually Transmitted Disease — a disease caused by the infection of a bacteria or virus transmitted through sexual contact.
With regard to HIV
These definitions class HIV as an infection. This is a fact to me because:
(bold emphasis mine)
Over time, HIV can destroy so many of these cells that the body can’t fight off [other] infections and disease. These opportunistic infections or cancers take advantage of a very weak immune system and signal that the person has AIDS, the last stage of HIV infection (HIV.gov, 2017)*
* Website managed by the U.S. Department of Health & Human Services and supported by the
Secretary’s Minority AIDS Initiative Fund (SMAIF).
HIV is a retrovirus, which is:
a family of viruses that are grouped together based on how they are structured and how they replicate within a host. Besides human immunodeficiency virus (HIV), the virus that causes AIDS, there a two other retroviruses that can cause human illness. One is called human T-lymphotropic virus type 1 (HTLV-1) and the other is called human T-lymphotropic virus type 2 (HTLV-II) (Minnesota Department of Health, n.d.).
Human T-cell lymphotropic virus type 1 (HTLV-1) is primarily transmitted through sexual intercourse or from mother to child (Paiva & Casseb, 2014) making HTLV-1 an STI in definitions I have been led to believe, and AIDS is an STD due to the fact that it is caused by HIV infection which in many circumstances is transmitted through sexual contact, although HIV can be transmitted through other means (see my answer). It cannot be an STI as AIDS is not transmitted. It is a disease as a result of HIV infection.
What creates a confusion?
In the comments to my answer, @Jan stated that HIV is not an STI, but just a virus. To make matters worse, when trying to confirm or deny his responses I have been trying to look up a definitive definition and found that in the key points of this factsheet, it states that:
HIV is an STD. Chlamydia, gonorrhea, human papillomavirus (HPV) infection, and syphilis are examples of other STDs. (National Institutes of Health, n.d.)
So the question is: What is the difference between STIs, STD and "just a virus transmitted through sexual contact"?
HIV.gov (2017) What Are HIV and AIDS?
Retrievable from: https://www.hiv.gov/hiv-basics/overview/about-hiv-and-aids/what-are-hiv-and-aids
Minnesota Department of Health, (n.d.) Other Retroviruses That May Cause Human Illness
Retrievable from: http://www.health.state.mn.us/divs/idepc/diseases/retrovirus/index.html
National Inststutes of Health (n.d.) HIV and Sexually Transmitted Diseases
Retreivable from: https://aidsinfo.nih.gov/understanding-hiv-aids/fact-sheets/26/98/hiv-and-sexually-transmitted-diseases--stds-
Paiva, A., & Casseb, J. (2014). Sexual transmission of human T-cell lymphotropic virus type 1. Revista da Sociedade Brasileira de Medicina Tropical, 47(3), 265-274.
DOI: 10.1590/0037-8682-0232-2013 PMID: 25075475