Introduction
Researchers are rapidly gaining knowledge about COVID-19 to help address
the current global pandemic, with a focus on treatment and prevention of
the spread of the disease[1–3]. This article has two primary aims.
The first aim is to examine the characteristics of the disease and the
individuals who are most susceptible to severe disease to see if they
can help reveal how humans can become less susceptible. A second aim is
to explore whether these considerations might suggest treatment
approaches that have potential to help at least some of those who are
already suffering from severe disease. It is hoped that this review will
be able to suggest areas of research that could be helpful in dealing
with both the current pandemic and with other similar diseases or future
epidemics.
The emphasis here will be on the human microbiome and the diet,
lifestyle and medical intervention factors that often affect it. This
emphasis arises from the increasing research showing the profound impact
of the human microbiome on immune function and many aspects of diverse
disease processes[4,5]. The human genetic makeup is certainly
important; however, the microbiotas inhabiting different parts of the
human body is increasingly being shown to be a crucial factor. It has
been estimated that there are as many bacterial cells in the human body
as human cells[6], and numerous microbial metabolites from the
microbiome reach the blood stream and are increasingly being
investigated[7]. This has led to the human microbiome being called
the second genome[8]. One advantage of focusing on this second
genome is that studies are showing that it can be altered much more
easily than the human genome, apparently with beneficial effects, such
as in the treatment of antibiotic-associated diarrhea due to
Clostridiodes dificile infection using fecal microbiota
transplants[9].
The rapid changes in diet, lifestyle, environmental exposures and
medical interventions in the last 75 years has led to changes in the
human microbiome that may not be optimally compatible with our evolved
immune responses to pathogens. This perspective is closely related to
the hygiene hypothesis[10] and its newer forms, such as the altered
microflora hypothesis[11] and an extension of these earlier
hypotheses that focuses on the potentially pathogenic microbes within
the post-hunter-gatherer era microbiota (PHM)[12]. Throughout this
review, attention will be paid to factors that might lead to the
establishment of these potentially pathogenic microbes, which include
environmental microbes that are less coevolved with their human hosts
and thus could have greater immune suppressing/dysregulating potential.