Guest Post by Dr Mariah Mosley, Image by Sascha Fritz from Pixabay 

We have all heard a lot lately about asymptomatic (symptom-free) carriers of infectious illness. This idea is based upon what’s called Germ Theory: the idea that germs are the enemy, to be directly attacked or avoided.

The opposite of Germ Theory is Terrain Theory: the idea that the health of the tissue (or the person) is what determines infection. 

A blend between the two also exists: this is the idea that it’s both the health of the tissue, combined with the virulence of the bug, that determines illness. 

Evidence for Asymptomatic Transmission

Over 10 years ago (2), researchers performed a systematic review of published studies looking at viral shedding and disease transmission for viral influenza. They “found that there is scant, if any, evidence that asymptomatic or pre-symptomatic individuals play an important role in influenza transmission. As such, recent articles concerning pandemic planning, some using transmission modeling, may have overestimated the effect of pre-symptomatic or asymptomatic influenza transmission. More definitive transmission studies are sorely needed.”

During the current pandemic, another study (1) exposed 455 people to asymptomatic COVID-19 carriers for 4-5 days, after which they were quarantined. When tested afterwards, no SARS-CoV-2 infections were detected by nucleic acid test. The conclusion was that “infectivity of the asymptomatic COVID-19 carriers might be weak”.

Please note that asymptomatic does not equal pre-symptomatic. Pre-symptomatic are the people who will develop symptoms, and the pre-symptomatic phase is about 5 days after exposure (5). Asymptomatic means no symptoms of the disease ever develop. 

The asymptomatic person is someone who is exposed to the virus, but nothing happens because their own innate immune system is able to fight it off immediately, without them even knowing they were exposed.

So what keeps the asymptomatic person free of symptoms?

Predisposition to Illness

Preexisting conditions have always been the biggest factor in the severity of acute infection. In one study (7) of 5,700 hospitalized COVID-19 patients in NY, 94% had a chronic health problem, and 88% had two or more chronic health problems. Hypertension, obesity, and diabetes were the three most prevalent conditions.

Other studies (8,9) have been done to prove this link to previous underlying chronic disease and poorer clinical outcomes, as well.

Germ vs Terrain

There are two very different schools of thought on why this might be. Germ theory rests on the ideas that disease arises from germs outside the body, microbes are generally to be guarded against, every disease is associated with particular microorganism, disease can strike anyone, germs as detrimental, and it doesn’t put an emphasis on our immune system (10). This theory was developed over 200 years ago by Louis Pasteur, credited with the discovery of microbes.

But we have learned a lot more about microbes in the last 200 years. For example, we now know we have as many bacteria in our bodies as we have of our own cells (12). Most of these bacteria are beneficial and actually help us—our gut microbiome, for instance, functions to maintain our gut integrity, assists with nutrient absorption, and helps us thrive (13). Scientists also recently found that we have a “virome”: a system of 300 trillion viruses that are beneficial for us and protect us. These viruses have a complex but beneficial interaction with humans (16), can positively up- or down-regulate our immune response and susceptibility to infection (14), and some are even being studied for use as anti-cancer drugs (15)!

This brings me to Terrain Theory. Terrain Theory rests on these ideas (10): opportunistic microbes feed on damaged/dead cells (i.e. the terrain has to be damaged), or microorganisms can become pathogenic as the health of the host organism deteriorates. Unhealthy conditions encourage disease, so the focus should be upon strengthening overall health, and the immune system.

Another way to understand this: Dr. Rudolph Virchow, the Father of Modern Pathology, said, “If I could live my life over again, I would devote it to proving that germs seek their natural habitat – diseased tissue – rather than being the cause of dead tissue. In other words, mosquitoes seek the stagnant water, but do not cause the pool to become stagnant.” 

In practice I see this as well. For example, fungal overgrowth in the gut occurs due to lack of good bacteria (such as after antibiotics (17)). The imbalanced flora is what causes the “weeds to grow”- so it is not the germ (a small amount of yeast in the gut is normal), but the changes in the environment that allows it to overgrow and cause symptoms.

A Third Option: Germ-Terrain Duality

So it is really the germ that causes disease? Or is it the terrain that allows the germ to set up shop?

A third theory incorporates both: the germ-terrain duality. This theory argues that “microbes need to be guarded against, but not at the expense of the terrain. Disease is due to both germs and the person’s diseased state, both equally important. To prevent disease, we must build our defenses, and create an environment of health” (10).

This study (11) of malaria also points to the germ-terrain theory, showing how ”tobacco use and/or physical strain under the hot sun, which cause cell and tissue damage, have been implicated in the etiology of malaria fever under the germ-terrain duality theory.”

My Take:

The concept of merely avoiding or killing germs to be free of disease doesn’t present the whole picture of what we know of immunology.  Children exposed to germs at an earlier age (playing in the dirt, living in rural areas or on a farm, being around large families) are less likely to have allergies, asthma, atopic disorders, or colitis and other gastrointestinal illnesses (18,19, 20, 21). This is because not all microorganisms are our enemies.

Our focus should therefore shift from germs alone to strengthening overall health and immune function. Then our bodies will be capable of fighting off pathogenic foreign invaders and opportunistic infections alike, while maintaining a beneficial relationship with our healthy microbiome.


  1. A Study on Infectivity of Asymptomatic SARS-CoV-2 Carriers
  2. Does Influenza Transmission Occur from Asymptomatic Infection or Prior to Symptom Onset?
  3. Clinical Characteristics of 24 Asymptomatic Infections With COVID-19 Screened Among Close Contacts in Nanjing, China
  4. Rapid Asymptomatic Transmission of COVID-19 During the Incubation Period Demonstrating Strong Infectivity in a Cluster of Youngsters Aged 16-23 Years Outside Wuhan and Characteristics of Young Patients With COVID-19: A Prospective Contact-Tracing Study
  5. The Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application
  6. Antibody tests suggest that coronavirus infections vastly exceed official counts
  7. Presenting Characteristics, Comorbidities, and Outcomes Among 5700 Patients Hospitalized With COVID-19 in the New York City Area
  8. Comorbidity and its impact on 1590 patients with COVID-19 in China: a nationwide analysis
  9. Prevalence of comorbidities and its effects in patients infected with SARS-CoV-2: a systematic review and meta-analysis
  10. The Differences Between the Germ Theory, the Terrain Theory and the Germ Terrrain Duality Theory
  11. Etiology, Epidemiology and Therapeutic History of Malaria Validate Germ- Terrain Duality; Postulates Thereof
  12. Revised Estimates for the Number of Human and Bacteria Cells in the Body
  13. Introduction to the human gut microbiota
  14. Human Virome
  15. Viruses as anticancer drugs
  16. The virome in host health and disease
  17. Antibiotic Exposure as a Risk Factor for Fluconazole-Resistant Candida Bloodstream Infection
  18. Early exposure to germs and the Hygiene Hypothesis
  19. Too clean, or not too clean: the Hygiene Hypothesis and home hygiene
  20. Time to abandon the hygiene hypothesis: new perspectives on allergic disease, the human microbiome, infectious disease prevention and the role of targeted hygiene
  21. Asthma and Atopy in Rural Children: Is Farming Protective?