Reproductive Health

Sexually transmitted infections and their causative agents are listed in NCERT Class 12, Chapter 4 (Reproductive Health). Acquired immunodeficiency syndrome, or AIDS, is caused by the human immunodeficiency virus, which infects and progressively destroys helper T-lymphocytes, weakening the immune system, and it currently has no complete cure. Syphilis is incorrect because it is caused by the bacterium Treponema pallidum and is treatable with antibiotics, so it is neither viral nor incurable. Gonorrhoea is wrong as it is a bacterial infection due to Neisseria gonorrhoeae and responds to antibiotic therapy. Chlamydiasis is also incorrect because it is caused by the bacterium Chlamydia and is similarly curable with appropriate antibiotics. The defining clues in the question are a viral cause, attack on immune cells, and absence of a complete cure, all of which point uniquely to HIV. A consistency check confirms this: among the listed infections, only AIDS is viral and immune-targeting, making it the correct answer.