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What causes a swollen throat?

Viral infection
Most sore throats are triggered by a viral infection. These are infections caused by a virus, such as the common cold or the flu.

Other types of viral infections include:

mononucleosis, which is an infectious disease typically transmitted through saliva
measles, which is a contagious illness characterized by a distinct rash and fever
chickenpox, which is an infection that causes skin sores
croup, which is an infection of the larynx

Bacterial infection
A bacterial infection can also cause a sore throat. These types of infections include:

strep throat, which is an inflammation of the throat caused by the Streptococcus pyogenes bacteria
diphtheria, which causes throat inflammation
whooping cough, which affects the respiratory mucous membrane

Environmental factors
Not all sore throats are viral or bacterial. Several other things can cause throat pain.

If you’re allergic to mold, pet dander, pollen, or other irritants, exposure to these allergens can trigger postnasal drip. Postnasal drip occurs when excess mucus accumulates in the back of your throat. This accumulation can irritate your throat and cause pain or inflammation.
Dry air can make your throat feel raw and scratchy.
Smoking cigarettes or breathing in cigarette smoke can trigger persistent sore throats.
Yelling or too much talking can trigger throat strain.