Gut, Immunity and Environment

Immune Health: Colds, Flu and Allergies, The Complete Guide

Immune health explained: why some people catch every cold, how sleep, vitamin D and stress shape defenses, and why allergy seasons keep getting worse.

By Dr. Samia McCully, ND · Updated October 23, 2026

Immune health describes how well the body defends against infections while staying calm toward harmless substances such as pollen and food. Colds, flu and seasonal allergies sit at opposite ends of that balance: too little defense allows frequent infection, while too much reactivity produces allergy symptoms. In 2024, 25.2 percent of U.S. adults had a diagnosed seasonal allergy, according to the National Center for Health Statistics. Sleep, vitamin D, stress, diet and the home environment shape both sides of immune balance. An evaluation of the conditions Dr. McCully treats looks at these foundations when infections or allergies keep returning.

Common Immune Challenges

Common immune challenges fall into infections the body fails to fend off and reactions it mounts against harmless triggers. Five patterns appear most often:

Frequent colds and respiratory viruses, which adults catch about two to three times a year on average and which some people catch far more often.
Influenza, a more serious respiratory infection that spreads most in fall and winter.
Seasonal allergies, also called hay fever or allergic rhinitis, which caused diagnosed symptoms in 29.9 percent of women and 21.1 percent of men in 2021.
Lingering or repeated infections, such as sinus infections that keep returning or colds that last for weeks.
Asthma and sinus inflammation, often driven by the same allergic and environmental triggers.

Dr. Samia McCully knows the allergy side firsthand. She describes severe childhood allergies, including a day her face swelled almost shut after she was pushed into tall grass at a festival, and antihistamines that did not always work.

Immune Foundations

Immune foundations are the daily factors that determine how well defenses work. Five carry the strongest evidence:

Sleep. In a 2015 study in which 164 adults were exposed to a cold virus, those who slept 5 to 6 hours a night had about four times the odds of developing a cold compared with those who slept more than 7 hours.
Vitamin D. A 2017 BMJ analysis of 25 trials found that vitamin D supplementation lowered the odds of respiratory infection, with the largest benefit in people who started out deficient.
Diet and blood sugar. Diets heavy in added sugar and ultra-processed food raise inflammation, while vegetables, fruit, protein and fiber supply the nutrients immune cells use.
Stress. Chronic stress raises inflammation, disrupts sleep and weakens defenses against viruses.
Movement and time outdoors, which support circulation, sleep and vitamin D production.

Dr. McCully's own rules for the season: avoid added sugar and eat fruit in moderation, avoid dairy apart from a little butter, sleep 7 to 8 hours, and test vitamin D. She aims for vitamin D levels of 75 to 85 ng/mL in most of her patients, above the 30 ng/mL threshold many laboratories use.

Environmental Triggers

Environmental triggers add to immune load from outside the body. Five matter most:

Pollen and climate. A 2021 analysis of North American pollen data found seasons starting about 20 days earlier and pollen loads about 21 percent higher than in 1990.
Mold and dampness. The World Health Organization reports up to a 75 percent higher risk of respiratory symptoms and asthma in damp or moldy buildings, covered in the everyday toxins guide.
Dust mites and pets, concentrated in bedding, carpets and upholstery.
Air pollution and wildfire smoke, which irritate airways and intensify allergic reactions.
Crowded indoor spaces, where respiratory viruses spread most in colder months.

Evaluation of Immune Problems

Evaluation of immune problems looks for the reasons defenses fall short or overreact. A complete evaluation contains five elements:

An illness and allergy history, including how often infections occur, how long they last, seasonal patterns and family history.
Vitamin D and nutrient labs, including 25-hydroxyvitamin D, ferritin and a complete blood count.
Blood sugar markers such as fasting glucose, fasting insulin and A1C, since poor glucose control weakens immune responses.
Allergy evaluation, including IgE testing when the history suggests specific allergens.
A home and work environment review, looking for dampness, mold, dust and air quality problems.

Dr. McCully looks further when symptoms persist despite diet changes, at gut health, mold exposure, stress and how well the body clears waste. The gut connection runs deep, as the gut health guide explains.

Support Approaches for Immune Health

Support for immune health strengthens defenses against infection and calms overreaction to harmless triggers. The core elements:

Hand hygiene. Community handwashing programs reduce colds and other respiratory illness by about 16 to 21 percent, according to the CDC.
Vaccination. The CDC recommends an annual flu vaccine for everyone 6 months and older.
Sleep and stress care, including a consistent bedtime and daily practices such as breathing, meditation and gratitude journaling.
Diet changes that cut added sugar and lower inflammatory load.
Targeted nutrients and botanicals, starting with vitamin D when levels run low, and saline nasal rinses during allergy season.
Reducing allergen and mold exposure at home through moisture control, HEPA filtration and washing bedding weekly.

Dr. McCully reports that many patients see milder allergy symptoms after removing added sugar, gluten and dairy, sometimes enough that herbs and other treatments become unnecessary.

Sleep Shapes Resistance to Colds

Sleep length directly affects how likely a cold virus is to take hold. In the 2015 Prather study, adults who slept 5 to 6 hours had about four times the odds of a clinical cold after exposure, and those under 5 hours fared similarly, compared with those sleeping more than 7 hours. During sleep, the body produces immune signaling proteins and consolidates immune memory. Chronic short sleep leaves those processes incomplete.

Vitamin D Status Affects Respiratory Infections

Vitamin D supports the immune cells that line the airways. In the 2017 BMJ meta-analysis, supplementation reduced the odds of acute respiratory infection overall, and people with very low starting levels saw the largest benefit. Levels drop in winter, with indoor lifestyles and with sunscreen use. Testing identifies who benefits most from supplementation.

Allergy Seasons Are Getting Longer

Allergy seasons now start earlier and carry more pollen than a generation ago. The 2021 North American analysis attributed much of the change to a warming climate. Patients who never struggled before now report itchy eyes, congestion and fatigue in spring. Longer exposure means the timing of prevention matters more, starting before the season peaks.

Inflammatory Load Lowers the Allergy Threshold

Allergy symptoms appear when the body's total inflammatory load exceeds what it tolerates. Dr. McCully describes the body as a cup: each inflammatory trigger adds water, and symptoms appear when the cup overflows. Pollen often supplies the final drops, while foods, gut problems, stress, mold and poor sleep fill most of the cup. Lowering the triggers within reach, such as diet, leaves room for the pollen that remains out of reach.

Stress Weakens Immune Defenses

Chronic stress shifts the immune system toward inflammation while weakening its response to viruses. Stress also shortens and fragments sleep, adding a second hit. People under long-term stress catch more colds and recover more slowly. Daily stress practices, connection with others and enough sleep restore resilience.

Frequently Asked Questions

Why do some people catch every cold?

Frequent colds usually reflect a combination of high exposure and weaker defenses. Short sleep, low vitamin D, chronic stress, poor blood sugar control and allergy-inflamed airways all raise susceptibility. Close contact with young children and crowded indoor spaces raises exposure. Addressing both sides lowers how often illness takes hold.

Does vitamin D prevent colds?

Vitamin D supplementation modestly lowers the risk of respiratory infections, with the largest benefit in people who start out deficient. A 2017 BMJ meta-analysis of 25 trials found lower odds of infection overall and much lower odds among those with very low baseline levels. Testing shows who benefits most.

Does stress weaken the immune system?

Chronic stress weakens immune defenses and raises inflammation, making infections more likely and recovery slower. Stress also disrupts sleep, which independently raises cold risk. Daily stress practices such as breathing, meditation and time outdoors support immune resilience.

Why are allergies getting worse?

Allergy seasons are getting longer and more intense. A 2021 analysis of North American pollen data found seasons starting about 20 days earlier and pollen loads 21 percent higher than in 1990, with climate change a major contributor. Air pollution and indoor exposures add to the burden.

Does diet affect seasonal allergies?

Diet influences the body's overall inflammatory load, which shapes how strongly it reacts to pollen. Dr. McCully reports that many patients see milder allergy symptoms after removing added sugar, gluten and dairy. Controlled research on elimination diets for seasonal allergies remains limited.

Common Misconceptions About Immune Health

Misconception: Milk Increases Mucus

Studies in people infected with cold viruses found no link between milk intake and nasal secretions or congestion. Milk's texture coats the mouth and throat, which feels like mucus. Some individuals still report congestion with dairy, so personal observation guides individual choices.

Misconception: Boosting Immunity Always Helps

An overactive immune response causes allergies and autoimmune disease. The goal is a balanced immune system that responds to real threats and ignores harmless ones. Sleep, nutrition and stress care support balance rather than simple stimulation.

Misconception: Allergies Are Only About Pollen

Allergy symptoms reflect the total load of triggers, including dust mites, pets, mold, air pollution and the body's own inflammatory state. Reducing indoor triggers and inflammation lowers reactions to pollen.

Misconception: Cold Weather Causes Colds

Viruses cause colds. Winter raises transmission because people gather indoors, dry indoor air irritates airways, and vitamin D levels fall. Cold temperatures alone do not create infection.

Misconception: Supplements Replace Sleep and Vaccines

No supplement offsets chronic short sleep, and supplements do not replace vaccines. The CDC recommends an annual flu vaccine for everyone 6 months and older. Nutrients support the foundations rather than replacing them.

Ongoing Debates in Immune Health

Sugar and immune suppression remain under study. Small, older experiments found reduced white blood cell activity for several hours after large sugar loads. Larger modern studies focus instead on how high-sugar diets raise inflammation and blood sugar, which impair immunity over time.

Vitamin D target levels divide experts. Many laboratories and endocrinology groups treat 30 ng/mL as sufficient. Some integrative practitioners, including Dr. McCully, aim higher, at 75 to 85 ng/mL, on the view that higher levels better support immune function.

Local honey for allergies draws mixed evidence. The idea holds that small amounts of local pollen in honey build tolerance. Trials show inconsistent results, and many patients report benefit anyway.

Elimination diets for allergies remain untested at scale. Clinicians who remove sugar, gluten and dairy report fewer allergy symptoms in many patients. Controlled trials on seasonal allergies are scarce.

Herbal remedies and antihistamines compete as first-line options. Stinging nettle, quercetin and vitamin C have traditional use and limited trial support. Antihistamines have stronger evidence but cause drowsiness in some people.

Components of the Immune System

The immune system contains several connected parts. Mucous membranes form a component of the body's first barrier, trapping viruses and allergens. White blood cells form a component of the immune system that attacks invaders and remembers them. Histamine forms a component of the allergic response, released by mast cells when they detect a trigger. Vitamin D status forms a component of immune evaluation. Sleep forms a component of immune recovery, when signaling proteins and immune memory build. Strong immunity depends on these parts working in balance.

Summary

Immune health balances defense against infection with calm toward harmless triggers, and colds, flu and seasonal allergies mark the two sides of that balance. Sleep, vitamin D, diet, stress and the home environment shape both. Sleep shapes resistance to colds, allergy seasons are getting longer, and inflammatory load lowers the allergy threshold. An evaluation of the conditions Dr. McCully treats checks these foundations when infections or allergies keep returning.