Acute bronchitis is usually caused by viruses, typically the same viruses that cause colds and flu (influenza). Antibiotics don't kill viruses, so this type of medication isn't useful in most cases of bronchitis.

The most common cause of chronic bronchitis is cigarette smoking. Air pollution and dust or toxic gases in the environment or workplace also can contribute to the condition.

Risk factors

Factors that increase your risk of bronchitis include:

• Cigarette smoke. People who smoke or who live with a smoker are at higher risk of both acute bronchitis and chronic bronchitis.
• Low resistance. This may result from another acute illness, such as a cold, or from a chronic condition that compromises your immune system. Older adults, infants and young children have greater vulnerability to infection.
• Exposure to irritants on the job. Your risk of developing bronchitis is greater if you work around certain lung irritants, such as grains or textiles, or are exposed to chemical fumes.
• Gastric reflux. Repeated bouts of severe heartburn can irritate your throat and make you more prone to developing bronchitis.

Vaccines prevent infectious illnesses by stimulating the immune system to recognize and fight off specific pathogens before they can cause disease. Vaccines contain harmless components of the pathogen, such as inactivated viruses or bacteria, or even small pieces of the pathogen’s proteins. When these components are introduced into the body, the immune system identifies them as foreign invaders and produces antibodies to neutralize them. This "training" of the immune system allows it to respond more effectively if exposed to the actual pathogen in the future.

By creating immunity without causing illness, vaccines help protect both individuals and communities through a concept known as herd immunity. When a large portion of the population is vaccinated, the spread of infectious diseases is reduced, protecting those who cannot receive vaccines due to medical reasons, such as infants or immunocompromised individuals. Vaccination programs have been instrumental in controlling, and in some cases eradicating, dangerous infectious diseases like smallpox and polio.