When I was in high school, we did a classic experiment in biology class. We were given test tubes, sugar, and yeast. We then put water, the sugar, and the yeast in the tube and mixed them up. We capped each tube and let them rest, returning each day after to take a small drop of the liquid. We then recapped the tube and returned it to the rack. We put the drop on a microscope's slide and counted the number of yeast cells across the microscope's field of vision. The idea was to take a consistent sample each time and count the cells in that sample. We kept records of the number of yeast cells each day over a week.
Of course, the yeast cell population zoomed for the next few days. As we graphed the sample size (on paper in such primitive times), we charted the increase in the number of cells in the sample and were able to infer the growth of the whole yeasty population. At some point, I don't remember exactly when, the number of cells leveled off and began a stair-step decline. The path on our chart resembled a ball bouncing down a flight of stairs with a rise in number followed by a greater fall before another "bounce" followed by another fall. The yeast population was clearly headed for extinction as it used up the resources of food and oxygen in the closed system. Needless to say, the yeast was also producing carbon dioxide that contributed to its decline. The yeast population had grown to the carrying capacity of the environment, but because it was a closed environment with no replenishment of food or oxygen the yeast ran out of resources and its decline was accelerated by their own waste products.
As you can tell, this lesson has stayed with me for more than 40 years. At that time and since, I have viewed it as a way to consider our potential path as a species. When I was even younger, in grade school, we learned that there were 3 billion people on our planet. We are now closing in on 7 billion as I write this with predictions of up to 10 billion by the middle of this century. The question we face is, what is the carrying capacity of our planet for our species? It's not a simple question to answer as we have pluses and minuses. For example, on the plus side, our technologies have enabled us to grow more food than ever on the same acreage. On the minus side, we have limits on some resources needed to sustain us--such as fresh water. In some ways we pit our technologies against the finite resources of our earth to see if we can provide for our needs.
The above is a round-about way to say that we are now at a point in our journey where we need to start looking around corners to see health threats to us. Some may be visible but disputed such as global climate change. Others may be invisible and undetected such as zoonoses where infectious diseases move into the human animal population from the non-human animal population. As we extract resources from tropical forests and bring those areas into the global economy, we also sometimes transport infectious diseases along with the plants, animals, and minerals we extract. Because most of our species may never have seen those pathogens before, they pose significant threats to our health because we have no immunity to them.
In the natural order, we would develop those immunities only after great pain and loss on a societal scale. The Bubonic Plague of the 14th century decimated Europe, but left a residual genetic trait that persisted in those who survived the pestilence and passed it to their children. It seems to confer greater immunity to plague to those who possess it. Scientists have found that trait across European descendants, but not in modern non-European populations. However the cost in suffering was immense. It took 150 years for Europe's population to recover.
As a member of the human family, I'd prefer to avoid such suffering. What we need is a societal immune system for our species.