The Claim

The claim says that regularly exposing the body to cold—through cold showers, ice baths, winter swimming or cryotherapy—“boosts” the immune system and therefore prevents colds, flu or infections generally.

The phrase “boosts immunity” is vague. This investigation focuses on the clinically meaningful version: people who repeatedly use cold exposure should actually develop fewer infections or fewer days of infectious illness.

Where It Comes From

Cold bathing has long cultural traditions, and modern cold-exposure communities often combine personal reports of resilience with studies showing changes in stress hormones, inflammatory markers or immune cells.

The idea also overlaps with the Wim Hof Method, where cold exposure is one component alongside breathing and meditation-like attention practices. Evidence for a combined protocol should not automatically be assigned to cold alone.

Why It Sounds Convincing

Cold exposure is a strong physiological stimulus. Heart rate, vascular tone, catecholamines and inflammatory signals can change rapidly. Adaptation to repeated cold is also measurable.

Because the immune system responds to stress, a changed immune marker can easily be described as “stronger immunity.” But immune function is not one dial, and a laboratory marker does not automatically predict fewer infections.

The Short Answer

Cold exposure has real physiological effects, but current evidence does not establish that it broadly prevents infections.

A large randomized cold-shower trial found fewer days absent from work but did not find fewer days of illness. A 2025 systematic review found a limited evidence base and no clear pooled immediate or one-hour immune-function benefit after cold-water immersion.

This leaves room for specific benefits while falling short of the broad infection-prevention claim.

How It Would Have to Work

Repeated cold exposure could plausibly alter stress signaling, immune-cell trafficking, inflammation or metabolism. To support the public claim, those changes should translate into fewer objectively defined infections or clinically meaningful reductions in illness.

A convincing study would compare a defined cold protocol with an appropriate control over enough time to capture infection incidence, while accounting for behavior, exercise, sleep and self-selection.

What Actually Happens

Acute cold activates the sympathetic nervous system, constricts peripheral blood vessels and alters stress hormones. Immune cells and inflammatory mediators can shift transiently as part of that stress response.

Repeated exposure may produce adaptation, but adaptation to cold is not synonymous with enhanced resistance to viruses or bacteria. Some immune responses protect against infection; others regulate inflammation; more activity is not always better.

What the Evidence Shows

In the randomized 2016 Dutch study, 3,018 adults were assigned to 30, 60 or 90 seconds of cold showers or a control condition. When the three cold-exposure groups were pooled, the adjusted incidence rate ratio for self-reported sickness absence was 0.71 (95% confidence interval [CI] 0.56–0.89), corresponding to 29% less reported work absence. But the incidence rate ratio for illness days was 0.89 (95% CI 0.77–1.01): its interval crossed 1, so the study did not demonstrate fewer illness days. Nor did it directly establish a reduction in laboratory-confirmed infections. Different cold-shower durations did not produce a clear dose-response gradient. Behavioral or attendance changes are plausible alternatives to immune protection.

The 2025 systematic review and meta-analysis found that cold-water immersion affects stress and inflammatory measures, but the evidence base was small and heterogeneous. It did not establish broad infection prevention.

The Strongest Evidence for the Claim

The strongest evidence is that cold exposure clearly changes physiology and that the randomized cold-shower trial found a meaningful reduction in sickness absence. Those findings make it reasonable to investigate health and behavioral effects further.

Some small studies also report shifts in immune markers. The limitation is that marker changes do not tell us whether participants actually resist infection better.

The Strongest Evidence Against It

The absence of reduced illness days in the large randomized trial is important because it separates “went to work less often while sick” from “was sick less often.”

The systematic review also shows that the direct clinical evidence remains too thin for a broad preventive recommendation based on immunity.

Common Viral Arguments

“Cold exposure increases immune cells.”

A temporary cell-count change can reflect redistribution rather than stronger pathogen defense.

“People who winter swim say they never get sick.”

Winter swimmers may differ in exercise, health, lifestyle and self-selection. Testimonials cannot isolate the cold exposure.

“The randomized trial showed 29% less sickness.”

It showed reduced sickness absence, not a significant reduction in illness days. Those are different outcomes.

“Inflammation went down, so immunity improved.”

Lower inflammation is not automatically stronger antimicrobial immunity. Immune regulation is context-dependent.

The Reasoning Error

The central error is treating a surrogate endpoint—an immune marker or stress response—as if it were the clinical endpoint of infection prevention. Correlation-causation error appears when healthier people who choose cold exposure are compared informally with everyone else.

Cherry-picking can promote the sickness-absence result while omitting the unchanged illness duration.

What Would Change Our Conclusion?

Large randomized studies showing fewer laboratory-confirmed or clinically defined infections in cold-exposure groups would materially change the conclusion. Replication across populations and clear protocol definitions would increase confidence.

Evidence that a specific immune change mediates the reduced infection rate would also strengthen the proposed mechanism.

Evidence Status

Status: Unsupported
Evidence quality: Moderate

Cold exposure produces real physiological adaptation and may affect wellbeing or behavior, but current evidence does not show that it broadly prevents infections through a generalized immune boost.

Sources

This investigation prioritizes the large randomized cold-shower trial and the 2025 systematic review. The evidence is interpreted separately for brief showers, whole-body cold-water immersion and broader cold-exposure practices rather than assuming all protocols are interchangeable.

Corrections & Updates

October 9, 2026 — Evidence interpretation update: clarified that the 2016 randomized cold-shower trial found fewer sickness-related absences (incidence rate ratio [IRR] 0.71) but no statistically significant reduction in days of illness (IRR 0.89); prevention of infections was not demonstrated (digital object identifier [DOI] 10.1371/journal.pone.0161749). Evidence Status unchanged.

Last Reviewed

October 9, 2026.