Humans have always faced disease outbreaks, sometimes spreading widely as pandemics. Dealing with these, reducing their frequency, and reducing harm when they occur are important reasons why we now live longer than our ancestors. As human society has progressed, we have become very good at managing risk and harm. A reduction in inequality and evidence-based health policies have been central to this success. Understanding how we got to this point, and the forces that are pulling us back, is vital to maintaining this progress. 

The World Around and Within Us

Infectious disease outbreaks happen. They once defined much of life, removing half the population in childhood and sometimes coming in waves that killed up to a third of entire populations. These historic outbreaks and life-shortening endemic diseases were mostly caused by bacteria, spread through poor hygiene and living conditions. Since we (re-)invented underground sewers, and (re-)understood the importance of clean drinking water and a good diet, mortality has greatly declined. We now live, on average, much longer. The development of modern antibiotics brought another huge step forward – most deaths during the Spanish flu, before modern antibiotics were invented, were due to secondary bacterial infections. 

Viruses also kill people directly and have devastated populations that had been relatively isolated for thousands of years. Measles and smallpox came close to wiping out whole populations, such as those of Oceania or the Americas, at the beginning of the European colonial era. But now, with perhaps the exception of HIV and respiratory viruses in the very frail elderly, the risk to most of us is low. Vaccination has further reduced this risk, but the vast bulk of reduced mortality in the wealthy occurred well before they became available for most vaccine-preventable diseases. This fact was once taught routinely in medical schools when evidence-based medicine was a primary driver of policy. 

Humans have evolved to live with bacteria and viruses, both friendly and harmful. Our ancestors have been dealing with them, in different variants, for hundreds of millions of years. We even contain descendants of simple bacteria within our cells – our mitochondria – containing their own genome. They and our far, far distant ancestors found a happy symbiosis where we protect them, and they provide energy for us. 

We also harbor billions of ‘foreign’ cells within our body – most of the cells we carry are not human but have a completely different genome. They are bacteria living in our guts, on our skin, and even within our blood. They are not an enemy – without some of them, we would die. They help us break food into forms we can absorb, they produce or modify essential nutrients, and they protect us from bacteria that would kill us if left unchecked. They produce chemicals that allow our brains to think critically and face the outside world with humor. Our bodies are a whole ecosystem in themselves, an unbelievably complex and beautiful symphony of life that sustains our being and gives a home and face to our spirit.

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