Causes of Diphtheria

Diphtheria is caused by the bacterium Corynebacterium diphtheriae. Not every strain of this bacterium causes disease. The dangerous ones carry a gene, picked up from a virus that infects the bacteria, that lets them produce diphtheria toxin. It is this toxin, not the germ itself, that destroys throat tissue and later injures the heart and nerves.

How it spreads:

• Droplets. When an infected person coughs, sneezes or talks closely, the bacteria travel in respiratory droplets to people nearby.
• Close contact. Sharing a household, classroom or sleeping quarters with a case or a carrier drives most transmission. Overcrowding is a major risk factor.
• Carriers. Some people carry the bacteria in their throat without any symptoms and can spread it for weeks. After recovery, a patient can also remain a carrier until follow-up tests are clear.
• Contaminated objects and skin lesions. Less commonly, the bacteria spread through shared items or from infected skin sores, which are more common in tropical climates.
• Related species. Rarely, Corynebacterium ulcerans, acquired from raw (unpasteurized) milk or close animal contact, produces the same toxin and the same disease.

Who is at risk:

• Unvaccinated or partially vaccinated children carry by far the highest risk. Nearly every diphtheria death in India in recent years was in a child who had not completed the vaccine series.
• Adults whose immunity has waned. Vaccine protection fades over the years, which is why booster doses (Td) are recommended in adolescence and adulthood.
• People in crowded or low-coverage communities, where one case can seed an outbreak.
• Those with weak immunity from illness or malnutrition.

One point matters more than the rest: diphtheria is not a disease of fate or bad water. It is almost entirely a disease of missed vaccinations. The cause is a bacterium, the protection is a vaccine, and the two facts belong together.