One study in Cell says "microorganisms are increasingly recognized as ecosystem-relevant components because they affect the population dynamics of hosts," suggesting a four-tier influence or "disease quadrangle" by host, host microbiome, pathogen, and environment (1). This report recommends better understanding the role of microbiomes in disease dynamics. A Medical News Today article implies the traditional "disease triangle" may be obsolete. For example, in the case of malaria, a mosquito is less likely to become vulnerable to the malaria parasite if it has more bacteria belonging to the Enterobacteriaceae family in its gut (2). The Enterobacteriaceae species protects against malarial infection. Researchers have found that climate temperature has a strong effect on the growth of pathogens and gut bacteria, and therefore the spread of disease.
Which brings us to our current COVID-19 problem. It is already known that gastrointestinal symptoms are common among people testing positive for COVID-19. A stronger link still as reported by Medical News Today say research found the presence of SARS-CoV-2 in the stool of COVID-19 patients (3). Though more research is required to conclusively prove these links, there is strong evidence that these links affect the severity and risk level of patients.
One point that can't be denied is that a significant paradigm shift has arrived with the fourth factor in how we look at disease and disease prevention. Host microbiome, is a critical part of understanding how we treat and prevent diseases.