Human Health And Disease

The Widal test is a serological test used to detect the presence of antibodies against Salmonella typhi, the causative agent of typhoid fever.
Hence, a positive Widal test indicates infection with typhoid.

Mast cells are immune cells found in connective tissues and play a critical role in allergic reactions.
They contain granules rich in histamine and other chemicals.
During allergic responses, allergens bind to IgE antibodies on mast cells, causing them to degranulate and release histamine.
This leads to symptoms like sneezing, itching, swelling, and mucous secretion.
Other options:
- Typhoid is a bacterial infection caused by *Salmonella typhi*; mast cells are not directly involved.
- Ascariasis is caused by a parasitic roundworm (*Ascaris lumbricoides*) and may activate eosinophils rather than mast cells.
- AIDS is caused by HIV which destroys helper T-cells, not primarily involving mast cells.

Elephantiasis is a disease caused by filarial worms like *Wuchereria bancrofti*, which are transmitted by mosquitoes.
These worms reside in the lymphatic system and cause chronic inflammation leading to enlargement of body parts, especially the legs.
Thus, it is a mosquito-borne disease that affects the lymphatic vessels.

Step 1: Both Streptococcus pneumoniae} and Haemophilus influenzae} are known to cause pneumonia and other respiratory infections.
Step 2: These bacteria spread through droplets and aerosols from coughing or sneezing.
Step 3: Hence, both the assertion and reason are true, and the reason explains the assertion correctly.