**Explanation:**

Diphtheria is an infectious disease caused by the bacterium Corynebacterium diphtheriae. This bacterium is responsible for causing diphtheria, a highly contagious disease that primarily affects the respiratory system but can also affect the skin.

**Bacteria as the cause:**

Diphtheria is caused by bacteria, specifically the bacterium Corynebacterium diphtheriae. This bacterium is rod-shaped and gram-positive. It produces a potent toxin called diphtheria toxin, which is responsible for the symptoms and complications of the disease.

**Transmission:**

Diphtheria is primarily transmitted through respiratory droplets from infected individuals. When an infected person coughs or sneezes, tiny droplets containing the bacteria can be inhaled by others, leading to infection. It can also be spread by direct contact with respiratory secretions or by touching contaminated surfaces and then touching the mouth, nose, or eyes.

**Symptoms:**

The symptoms of diphtheria can vary depending on the site of infection. In respiratory diphtheria, the bacteria infect the throat and tonsils, causing symptoms such as sore throat, fever, and difficulty breathing. In cutaneous diphtheria, the bacteria infect the skin, leading to skin ulcers or sores.

**Complications:**

If left untreated, diphtheria can lead to severe complications. The diphtheria toxin produced by the bacteria can cause damage to various organs, including the heart and nervous system. It can lead to myocarditis (inflammation of the heart muscle), nerve damage, paralysis, and even death.

**Prevention and Treatment:**

Diphtheria can be prevented through vaccination. The diphtheria vaccine is commonly administered as part of the routine childhood immunization schedule. It is usually given in combination with vaccines for tetanus and pertussis (whooping cough), known as the DTaP vaccine.

Treatment for diphtheria involves the administration of antibiotics to kill the bacteria, as well as antitoxin to neutralize the diphtheria toxin. It is important to seek medical attention promptly if diphtheria is suspected, as early treatment can prevent complications and reduce the risk of spreading the infection to others.

In conclusion, diphtheria is caused by the bacterium Corynebacterium diphtheriae, making option D (Bacteria) the correct answer.

Disease caused by bacteria...
tetanus, cholera, typhoid, tuberculosis, diphtheria, plague, whooping, pneumonia, gonorrhea, syphilis.... rattta maar lijiye.. jyda brief me na padhiye.. sufficient h etna