Mismatch: AIDS - Bacterial infection

Explanation:
AIDS (Acquired Immunodeficiency Syndrome) is caused by the Human Immunodeficiency Virus (HIV), which is a retrovirus and not a bacterial infection. Therefore, option B is a mismatch.

Details:

1. Leprosy - Bacterial infection:
Leprosy, also known as Hansen's disease, is caused by the bacteria Mycobacterium leprae. It primarily affects the skin, peripheral nerves, mucosa of the upper respiratory tract, and eyes. Leprosy is a chronic infectious disease that mainly spreads through respiratory droplets from an infected person.

2. AIDS - Bacterial infection:
AIDS is not caused by a bacterial infection but by the Human Immunodeficiency Virus (HIV). HIV attacks the immune system, specifically the CD4 cells (T cells), which are crucial for fighting off infections. As the virus destroys these cells, the immune system becomes weakened, making the person more susceptible to various infections and certain types of cancer.

3. Malaria - Protozoan infection:
Malaria is caused by protozoan parasites of the genus Plasmodium. It is transmitted through the bites of infected female Anopheles mosquitoes. The parasites multiply within the liver and then infect red blood cells, leading to symptoms such as fever, chills, headache, and fatigue.

4. Elephantiasis - Nematode infection:
Elephantiasis, also known as lymphatic filariasis, is caused by infection with parasitic nematodes (roundworms) Wuchereria bancrofti, Brugia malayi, or Brugia timori. These worms are transmitted to humans through the bites of infected mosquitoes. The infection primarily affects the lymphatic system, leading to the swelling and enlargement of body parts, particularly the limbs and genitals.

In conclusion, the mismatch in the given options is option B, as AIDS is caused by the HIV virus, which is a retrovirus and not a bacterial infection.