Faced with the threat of COVID-19, most countries in the world are suddenly, dramatically improving their hygiene and infectious disease control. This won't just slow the spread of COVID-19. As a byproduct, it will also suppress almost every other contagious disease. This creates a natural experiment, which answers the question:
What would happen if we fully suppressed influenza, the common cold, and all the other ignored, minor infectious diseases, all at once?
About 13% of cancers worldwide are attributable to known infectious causes. Many other health problems are suspected to be related to infectious disease, including chronic fatigue syndrome, type 1 diabetes, hypothyroidism, obesity, Alzheimers disease, bipolar disorder, and some mechanisms of aging. (And obesity again but this time it's a bacterium instead of a virus.)
In the coming year, we're going to see large reductions in many diseases we never realized were infectious-disease related. As bad as COVID-19 is, this will be a pretty substantial silver lining.
Most of the diseases won't stay gone; social distancing and citywide lockdowns are costly, and most diseases will retain enough reservoirs to bounce back after we return to normal. What we'll get is a period we can study retrospectively, different in different countries, in which almost no disease transmission occurred.
This will be confounded by COVID-19 itself, of course. People who themselves had COVID-19 will have a lot of health problems, which studies will need to account for. The isolation of social distancing, the anxiety of watching the disaster unfold, and the poverty of job-less and economic decline will leave their mark. In countries where COVID-19 took hold, during the height of the pandemic, cancers and autoimmune disorders and other conditions will have been left undiagnosed due to the shortage of hospitals. We won't easily be able to tell which diseases matched with which health conditions. With clever methodology and access to data in countries which had different outcomes, all of these issues will be possible to work around.
The world's attention is now very focused on the short term. Let's give a little thought to what next year will look like.