Early life is a critical time for immune system development. Research shows that regular exposure to everyday microbes helps train the immune system to respond appropriately and reduces the risk of allergies and asthma later in life. Early research showed that the common cold can possibly act as a deterent to allergies and asthma later in life [1].

Studies have found that children who grow up with pets, farm animals, or outdoor environmental exposure are less likely to develop asthma and allergic diseases. These exposures appear to help the immune system learn the difference between harmless substances and real threats. This is the source of the “hygene hypothesis”, which has been extensively studied [2-4].

At the same time, newer research shows that severe viral infections of the lower respiratory tract in infancy, especially certain rhinovirus and RSV infections, are associated with a higher risk of asthma. These infections may damage developing airways or reveal an underlying tendency toward asthma rather than protect against it [].

Taken together, the evidence suggests that immune training comes from diverse, low-level microbial exposure, not from severe or repeated serious infections. An immune system that develops in a balanced microbial environment is less likely to overreact, while significant early lung infections may increase asthma risk in susceptible children.

References:

  1. British Medical Journal, (February 17, 2001; 322: 390-395) Early childhood infectious diseases and the development of asthma up to school age: a birth cohort study
  2. The ‘hygiene hypothesis’ for autoimmune and allergic diseases: an update. Clin. Exp. Immunol. 2010;160:1–9. 
  3. Exposure to environmental microorganisms and childhood asthma. N. Engl. J. Med. 2011;364:701–709. 
  4. Family size, infection and atopy: the first decade of the “hygiene hypothesis”. Thorax. 2000;55(Suppl. 1):S2–S10. 
  5. Hospitalization for RSV bronchiolitis before 12 months of age and subsequent asthma, atopy and wheeze: a longitudinal birth cohort study. Pediatr. Allergy Immunol. 2005;16:386–392.
  6. BMC Pediatr. 2005 Aug 18;5:31. Acute bronchiolitis in infancy as risk factor for wheezing and reduced pulmonary function by seven years in Akershus County, Norway