Answer :

It'd be the second answer, a virus, which attacks the white blood cells.

Virus  

Further Explanation:

AIDS (acquired immune deficiency syndrome) is caused by HIV (human immunodeficiency virus). The virus is a retrovirus that has a cone-shaped core that contains two copies of the single-stranded genome, reverse transcriptase enzyme, and core proteins. HIV infection weakens the immune response of the host body due to which the host becomes susceptible to secondary infections like fever, diarrhea, and cold. The infected person is at a higher risk of opportunistic pathogens. The population of T-helper cells declines during HIV infections. AIDS is the ultimate stage of HIV infection, where the host has a severely damaged immune system.  

HIV infection is transmitted through body fluid (semen, vaginal fluid, breastfeeding, and blood) from an infected individual. The infection is spread through sex with multiple partners, sharing infected needles and syringes, and blood transfusion. The infection is not transmitted through insect bite, air, and water. The infected person may remain asymptomatic for a long time. The HIV infection can be diagnosed by ELISA (enzyme-linked immunosorbent assay) test.  

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Answer Details:  

Grade: Senior School

Subject: Biology

Topic: Viruses

Keywords:

Virus, retrovirus, reverse transcriptase, core proteins, opportunistic pathogens, single-stranded genome, fluids, semen, blood, ELISA.