Public health guidance often focuses on avoiding exposure to viruses — washing hands, limiting contact during outbreaks, and maintaining good hygiene. These steps matter. But as Louis Pasteur reportedly said near the end of his life:
“Bernard was right… the pathogen is nothing; the terrain is everything.”
Your “terrain” is your internal environment — the strength, flexibility, and resilience of your immune system.
Stress and Viral Susceptibility
Stress can weaken the immune system. Chronic stress can weaken several aspects of immune function, including antibody production, NK-cell activity, and mucosal defenses.
A study in Psychosomatic Medicine (May 2001) [1] found that students experiencing higher stress and pain scores were more likely to develop cold or flu symptoms than students reporting lower stress. This aligns with modern research showing that chronic stress elevates cortisol and inflammatory cytokines while impairing the innate immune response.
Nutrition and the Immune System
The Western diet and inflammation
Diets high in sugar, refined carbohydrates, and processed fats can promote chronic inflammation and disrupt immune signaling [2]. In short, the Western diet undermines your immune system [3,4].
A review in Frontiers in Immunology (2020) highlighted that high-sucrose, low-fiber diets reduce microbial diversity and increase pro-inflammatory cytokines — weakening both gut and respiratory immune defenses.
Polyphenols as immune modulators
Polyphenol-rich foods (berries, green tea, turmeric, cocoa, herbs/spices) have long been studied for their antioxidant and immunomodulatory activity. A review in Nutrients (2020) found that polyphenols may support immune function by influencing cytokine balance, regulating immune cell signaling, and protecting tissues from oxidative stress [5].
Vitamin C and Viral Immune Support
Vitamin C plays a well-documented role in immune cell function, antioxidant protection, and epithelial barrier support.
Older clinical studies
The large student trial (J Manipulative Physiol Ther. 1999) found that high-frequency vitamin C dosing reduced reported respiratory symptoms by 85% compared with controls. Another trial (CMAJ. 1975) found that 500–1,500 mg/day shortened the duration of respiratory symptoms.
Newer evidence
Modern meta-analyses provide clearer context:
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A 2013Cochrane review found that vitamin C may reduce the duration and severity of viral upper respiratory infections [6].
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Research in Nutrients (2017) reported that vitamin C supports epithelial barrier function and enhances neutrophil activity in early viral defense [7].
Vitamin A and Mucosal Immunity
Vitamin A is essential for mucosal immunity—the body’s first barrier against viruses. Adequate vitamin A supports epithelial integrity and IgA production, and human studies link good vitamin A status with improved outcomes in viral infections such as measles, RSV, HIV, and Ebola [8–12]. Click to read more about vitamin A.
Vitamin A + D synergy
A 2019 study in Viruses reported that vitamin A and D supplementation improved vaccine responses in children who were insufficient at baseline [13].
Probiotics, Gut Ecology, and Viral Immunity
The gut houses a large portion of the immune system (the GALT). Probiotics can influence antibody production, inflammatory tone, and response to viral challenges. Click to read more about probiotics and immunity.
Human clinical studies
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A study in 140 stomach cancer patients found that probiotics increased IgG, IgA, and IgM while improving inflammatory cytokine profiles [14].
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Probiotics enhanced the effectiveness of the flu vaccine in another randomized controlled trial [15].
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A 2018 systematic review and meta-analysis found that probiotics can modestly reduce the incidence and duration of upper respiratory infections in both adults and children [16].
Aging and immunity
Research in Mechanisms of Ageing and Development (2018) reports that probiotics can reduce inflammation associated with aging (“inflammaging”), potentially improving immune balance.
Building a Strong Terrain
Supporting immune resilience is not about treating disease — it is about strengthening the “terrain”:
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Reducing chronic stress
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Eating nutrient-dense, polyphenol-rich foods
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Ensuring adequate vitamins A, C, and D
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Supporting a healthy microbiome through fiber and probiotics
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Maintaining balanced lifestyle habits (sleep, movement, sunlight)
These factors work together to support balanced immune function year-round — and may help the body respond more effectively to viral challenges.
In Pasteur’s words:
“…the terrain is everything.”
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Immunity. 2019 Nov 19;51(5):794-811. Western Diet and the Immune System: An Inflammatory Connection
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J Immunol Res. 2018 Apr 12:2018:1264074. Regulation of Immune Function by Polyphenols
- 2013 Cochrane review
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Nutrients. 2017 Nov 3;9(11):1211. Vitamin C and Immune Function
