The infectious agents causing pneumonia may reach the person's lungs through different routes. The most common modes of transmission are:
- Airborne droplets spread when the sick person coughs or sneezes, and inhaled into the lungs (breathed in) by a susceptible person;
- Direct oral contact with someone who has pneumonia (e.g. through kissing).
- During or shortly after birth, babies are also at higher risk of developing pneumonia from coming into contact with infectious agents in the birth canal, or from contaminated articles used during the delivery.
These modes of transmission help to explain why certain risk factors increase the probability that children or adults will develop pneumonia. Box 35.3 summarises the common risk factors, some of which also increase the patient's susceptibility to fatal complications if pneumonia occurs.
Box 35.3 Common risk factors for pneumonia
- Under-nutrition/malnutrition, which weakens the immune system and reduces resistance to infection
- Inadequate breastfeeding or formula feeding of infants under six months old, which predisposes them to malnutrition and infection
- Lack of immunization against vaccine-preventable diseases that affect the respiratory system
- Infection with HIV and/or tuberculosis
- Living in overcrowded homes, where airborne infection is easily transmitted
- Exposure to indoor air pollution, especially smoke from cooking fires burning vegetable and animal waste (e.g. dried cow dung), which irritates the lungs and makes it easier for bacteria and viruses to gain a hold.