**Core Concept**
Trachoma is a chronic keratoconjunctivitis caused by the bacterium **Chlamydia trachomatis**. It is a leading cause of preventable blindness worldwide, particularly in areas with poor sanitation and hygiene.

**Why the Correct Answer is Right**
Trachoma is a contagious disease that spreads through direct or indirect contact with infected secretions from an infected person. The bacterium **Chlamydia trachomatis** serovars A, B, Ba, and C are responsible for the disease. Infection occurs when the bacteria enter the eye through the upper conjunctiva, leading to inflammation and scarring of the cornea. If left untreated, trachoma can cause permanent vision loss.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as trachoma is not caused by a viral infection. While some viral infections can cause conjunctivitis, trachoma is specifically caused by a bacterial agent.
**Option B:** This option is incorrect as trachoma is not associated with a fungal infection. Fungal infections can cause a range of ocular diseases, but trachoma is a bacterial infection.
**Option C:** This option is incorrect as trachoma is not caused by a parasitic infection. While some parasites can cause ocular diseases, trachoma is specifically caused by a bacterium.

**Clinical Pearl / High-Yield Fact**
The World Health Organization (WHO) has developed a strategy to eliminate trachoma by the year 2020, which includes improving access to safe water and sanitation, promoting face washing, and providing antibiotic treatment to infected individuals.

**Correct Answer:** D.