Epidemiologists Judith Glynn and Hilary Bower, at the London School of Hygiene and Tropical Medicine, have conducted several large-scale studies into infectious diseases in Africa. During Ebola virus outbreaks, they also found exposure-adjusted relative risks to be the lowest for those in the 10-to-14 age group.

Although the estimation of mortality and morbidity rates caused by pandemics is fraught with difficulty, a trend seems to emerge from current and previous pandemics. Prepubescent and post-pubescent children seem to show a lowered risk for viral infections than the rest of the population.

Has evolution pronounced this age group to be the fittest among us? Is it because we need this demographic to reproduce and perpetuate our species, and prevent us from becoming extinct? And if so, how has evolution protected this demographic from virulent attacks?

Norwegian scientists John Moxnes and Olav Christophersen explain that the immune system is strongly influenced by hormones, including reproductive hormones. They speculate the involution (shrinkage) of the thymus starting at puberty to be one of the reasons why several infectious diseases appear to be less virulent and less lethal when affecting children than they do when they affect adults who have not been exposed to the same infectious agent before.

“The thymic hormones help to stimulate both innate and adaptive immunity, while the combination of larger thymic hormone production and much larger thymic production of T cells in children will make it possible for the ‘rescue forces’ to arrive earlier and be strong and numerous when they arrive,” they write.

Molecular geneticist and Nobel laureate Joshua Lederberg, in his famous paper “Pandemic as a Natural Evolutionary Phenomenon”, said we fail to acknowledge our relationship to microbes as a continued evolutionary process that needs to be in equilibrium.

The phenomenal growth of Homo sapiens since the Paleolithic era is paralleled only by the phenomenal strides science and technology have made in increasing the longevity of our species. Consequently, this has become the major cause of disturbance to that hypothetical balance in our ecosystem.

Lederberg suggests we are complacent to trust that nature is benign; we are arrogant to assert that we have the means to except ourselves from the competition. But our principal competitors for dominion, outside our own species, are the microbes: the viruses, bacteria and parasites. They remain an interminable threat to our survival.

Samuel Alizon and Pierre-Olivier Méthot write that Pasteur’s work allowed him to demonstrate how, after several passages in new hosts, the virulence of a microbial strain can increase for these hosts and decrease for the original host. Pasteur claimed smallpox, syphilis, plague and yellow fever appeared across the ages because “by this method new virulences and new contagions can be created”.

Governments have been criticised for being inadequately prepared for this pandemic, and we are learning to be better prepared for the next one. However, Alizon and Méthot suggest that the continual emergence of new pathogens and the increased spread of antibiotic resistance in bacterial populations should serve as reminders that microbes are living entities that evolve at rates which affect public health interventions.

Although Pasteur and Darwin apparently never met during their lifetime, we must teach our students the need to reconcile their work in clinical microbiology and evolutionary biology to understand the significant threat microbes pose to us.

Anjali Hazari is a retired international-school biology educator who has taught for three decades in Hong Kong and has received several accolades in her teaching career. She continues to tutor and write extensively on education policy and practice.