What does Cold, Flu & Allergy do in the body?

Story

Cold, Flu & Allergy

Last updated September 2026

In one pass A blocked, runny nose can come from three completely different mechanisms: rhinoviruses slowly copying themselves in the lining of the nose; influenza viruses hitting the whole body suddenly; or the immune system…

Educational content, not medical advice — consult a clinician.

Chapter 1

Telling cold, flu and allergy apart

A blocked, runny nose can come from three completely different mechanisms: rhinoviruses slowly copying themselves in the lining of the nose; influenza viruses hitting the whole body suddenly; or the immune system treating pollen and dust mites as invaders, parking IgE antibodies on mast cells that release histamine the moment they meet the trigger (Wheatley & Togias 2015).

The common cold is mostly caused by rhinoviruses and some coronaviruses. Symptoms build gradually, mainly a runny or blocked nose and a scratchy throat, and fever is rare (CDC). Flu is caused by influenza viruses, and symptoms hit all at once: high fever, aches all over, exhaustion and headache, and you are sicker (CDC). Allergic rhinitis is not an infection: it brings bouts of sneezing, a clear watery runny nose and itchy eyes and nose, comes back with the surroundings, and causes no fever.

Three mechanisms, three ways of handling them. Whichever it is, if you have trouble breathing, bluish lips, or sudden swelling of the face or tongue, seek medical care immediately.

Mechanism · What each of the three does in the body

All three play out in the upper airway (the nose and throat), but the body is doing a completely different job in each.

Common cold: rhinoviruses and some coronaviruses copy themselves in the lining of the airway. Symptoms build gradually, mainly a runny or blocked nose and a scratchy throat; fever is rare and body aches are mild (CDC)Flu (influenza): caused by influenza viruses, with symptoms that hit all at once; high fever, aches all over, exhaustion and headache are all prominent, and you are sicker (CDC)Allergic rhinitis: not an infection at all. The immune system overreacts to things that are harmless in themselves, such as pollen and dust mites, bringing bouts of sneezing, a clear watery runny nose and itchy eyes and nose, coming back with the surroundings, and causing no fever (Wheatley & Togias 2015)
Telling these three apart is the first step to handling any of them correctly. Viral infections mostly clear up on their own; people at high risk from flu should see a doctor early; and allergy calls for avoiding the trigger and taking antihistamines, which points in the opposite direction from "boosting immunity".

Chapter 2

Fever and mucus fight the virus

When you have a cold or flu, much of what makes you feel bad, the blocked and runny nose, the low fever, the tiredness, is not damage done directly by the virus. It is innate immunity at work on the airway lining: cells release interferons and inflammatory signals, the body deliberately raises its temperature so the immune response runs more efficiently, and mucus flushes the virus out and traps it.

Next, adaptive immunity recognizes the virus and makes antibodies, clears it within a few days, and leaves a memory behind. That is why ordinary viral upper-respiratory infections mostly clear up on their own: symptoms are usually worst around days 2–3 and ease by themselves within 7–10 days, though in some people a cough lingers for up to 3 weeks (Heikkinen & Järvinen 2003).

Most colds need time, rest and fluids, not a miracle drug.

Mechanism · Innate immunity first, then adaptive

Fever and mucus are the body at work. The battlefield is the lining of the nose and throat, and two waves of defenders go in.

Innate immunity goes first: the moment a virus gets in, cells in the lining and immune cells release interferons and inflammatory signals. Fever is the body deliberately raising its temperature so the immune response runs more efficiently; mucus is the lining flushing the virus out and trapping it. Adaptive immunity follows: T cells recognize cells the virus has infected, B cells make antibodies against this particular virus, the infection is cleared within a few days, and a memory is left behind so the next response is faster.

Once you see the symptoms as the body working, you will not rush to suppress every runny nose with medicine. Letting immunity finish at its own pace usually takes those 7–10 days.

Chapter 3

Allergy is an immune overreaction

Allergic rhinitis looks like a cold, but the mechanism runs the other way: the immune system treats harmless pollen, dust mites or pet dander as a threat and makes IgE antibodies that sit on mast cells in the lining of the nose. At the next encounter, those cells release histamine within minutes; blood vessels widen and glands pour out fluid, and the result is fits of sneezing, a clear watery runny nose and itchy eyes and nose (Wheatley & Togias 2015).

That is why antihistamines help allergy but do little for a cold. Allergy is an immune system that is overactive and aimed at the wrong target, not one that is too weak; trying to treat allergy by "boosting immunity" points in exactly the wrong direction.

Mechanism · Why boosting immunity points the wrong way

A cold is the immune system fighting a real enemy (a virus); allergy is the immune system fighting a war it should never have started (Wheatley & Togias 2015).

In people prone to allergy, the immune system has made IgE antibodies against a particular allergen, and these sit on mast cells in the lining of the nose. At the next exposure, the allergen links neighboring IgE molecules together (cross-linking), and within minutes the mast cells release histamine and other substances: blood vessels widen and glands pour out fluid, bringing fits of sneezing, a clear watery runny nose and itchy eyes and nose. Antihistamines block exactly that histamine step.

What you want is to calm this false-alarm system down so it stops firing at nothing (antihistamines, nasal steroid sprays, avoiding the allergen), not to press its accelerator. Itchy sneezing that keeps coming back, follows the seasons or your surroundings, and comes without fever looks more like allergy than infection, and calls for a different approach; if you are unsure, or it is already affecting your life, see a doctor for proper treatment.

Watch the mechanism

Chapter 4

Do immune boosters work?

The immune system is not a dial where higher is always better: its way of failing when overactive is allergy and autoimmune disease, and a finely regulated system cannot be "upgraded" by taking something. For people who are genuinely short of a nutrient (zinc or vitamin D, for example), bringing it back to normal is worthwhile, but that is filling a gap, not boosting.

The products that sell hardest when the seasons change, vitamin C fizzy tablets, zinc lozenges, echinacea and elderberry, all land somewhere between no effect and a weak one when you check the evidence one by one. Enough sleep, not smoking, a balanced diet and keeping up with vaccinations are more reliable than any tube of fizzy tablets.

Evidence · Vitamin C and zinc

Start with the two that sell hardest.

Vitamin C (Hemilä & Chalker 2013, a Cochrane systematic review): in the general population, even long-term supplements do not prevent colds; regular supplements shorten a cold in adults by only about 8%; and starting once symptoms appear does essentially nothingZinc lozenges (Nault 2024, a Cochrane systematic review): they may shorten a cold by about two days, but the certainty of the evidence is low, and side effects such as nausea and a bad taste in the mouth are common
The wish to get sick less often is real, but it is not a matter of turning an immunity dial up a notch.

Evidence · Echinacea, elderberry and garlic

Echinacea (Karsch-Völk 2014, a Cochrane systematic review): 24 trials with over 4,600 people; at most a weak preventive effect of doubtful clinical relevance, and no reliable treatment effectElderberry: the evidence is weak and contradictory. Tiralongo 2016 ran a randomized trial in 312 air travelers: the drop in how many caught a cold was not statistically significant, though those who did catch one had a shorter, milder illness (low certainty); an in flu patients, Macknin 2020, saw no benefit at allGarlic (Lissiman 2014, a Cochrane systematic review): only 1 study met the inclusion criteria, too little evidence to conclude anything
Don't stake your money or your hopes on immune boosters. What really helps is vaccination, rest and fluids; see the chapter Vaccines and rest actually help.

Chapter 5

Vaccines and rest actually help

Against flu, the most effective move is to get vaccinated ahead of time, so the immune system builds a memory first, especially for high-risk groups such as older adults, people with chronic illness and pregnant women (CDC adult immunization schedule). This is not "boosting"; it is setting up the defenses in advance. The common cold has no specific cure: rest, fluids and relief of symptoms are enough, while the illness runs its self-limiting course (Heikkinen 2003).

Colds and flu are both caused by viruses, and antibiotics work only on bacteria. A Cochrane systematic review is clear: antibiotics bring no benefit for a cold, cause side effects such as diarrhea in about 1 in 10 people (Kenealy & Arroll 2013), and feed antibiotic resistance. Yellow or green mucus does not mean you need antibiotics.

Clinical · Who needs flu antivirals

There is one more move, only for the people who need it: flu antivirals such as oseltamivir help high-risk or severely ill patients, but they need to start early, ideally within 48 hours of the first symptoms, and a doctor decides (CDC). Otherwise healthy people with mild flu usually do not need them.

Put your attention on vaccination (the CDC adult immunization schedule), rest and fluids, and seeing a doctor when medication is warranted; do not take antibiotics on your own, and do not count on "immune-boosting" supplements.

Chapter 6

When to see a doctor

Most colds and flu get better on their own because the immune system clears the virus; but a persistent high fever, labored breathing, or getting worse after ten days means this fight has gone beyond ordinary defenses in the airway lining, and it is time for a doctor to take over.

A persistent high fever (for example, above 39°C and not coming down), or a fever that has not gone after 3–4 days; trouble breathing, chest pain, shortness of breath, or bluish lips; symptoms that have not improved after 10 days, or that improve and then suddenly get worse (a two-phase course that may mean a secondary bacterial infection): none of these are signals to tough out at home.

Red flag · More signs that need a doctor

Besides a persistent high fever, trouble breathing, and improving then suddenly getting worse, these signs also mean seeing a doctor:

Marked lethargy, confusion, severe dehydration, a severe headache, or a stiff neckInfants, older adults, pregnant women, and people at high risk such as those with chronic heart or lung disease, diabetes or a weakened immune system should seek care earlier when symptoms are significant
About allergy: if you develop trouble breathing, swelling of the face or tongue, hives all over the body, or dizziness (possibly a severe allergic reaction, anaphylaxis), get emergency help immediately.

Disclaimer: this page is education, to help you understand the mechanisms and make smarter choices; it does not replace a doctor's diagnosis. Whether to use antivirals or antihistamines, and how to get vaccinated, differs from person to person; if you have symptoms, take medication or belong to a high-risk group, consult a qualified doctor or pharmacist.

References · 12

Centers for Disease Control and Prevention. (2024). Cold Versus Flu. CDC. A cold comes on gradually with mild symptoms and rare fever, mainly runny/stuffy nose; influenza has abrupt onset with usual fever (3-4 days), prominent body aches, fatigue and headache, and is generally more severe. www.cdc.gov/flu/about/coldflu.html
Wheatley, L. M., & Togias, A. (2015). Allergic rhinitis. New England Journal of Medicine, 372(5), 456-463. Allergen cross-linking of IgE bound to mucosal mast cells triggers histamine release within minutes, producing early-phase nasal symptoms; allergic rhinitis is a misdirected IgE-mediated hypersensitivity, not a weak immune system. 10.1056/NEJMcp1412282
Heikkinen, T., & Jarvinen, A. (2003). The common cold. The Lancet, 361(9351), 51-59. Rhinoviruses are the most prominent cause; upper respiratory infections are generally mild, self-limiting and viral, with symptom duration typically 7-10 days (peak day 2-3). 10.1016/S0140-6736(03)12162-9
Hemilä, H., & Chalker, E. (2013). Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews, (1), CD000980. Regular vitamin C does not reduce cold incidence in the general population; it modestly shortens duration (~8% in adults), with no benefit when started at symptom onset. 10.1002/14651858.CD000980.pub4
Nault, D., Machingo, T. A., Shipper, A. G., et al. (2024). Zinc for prevention and treatment of the common cold. Cochrane Database of Systematic Reviews, (5), CD014914. Zinc may reduce cold duration by ~2 days but with low-certainty evidence; insufficient to recommend zinc for preventing or treating colds. 34 RCTs, 8,526 participants. Prevention: little or no effect on getting a cold (RR 0.93, 0.85-1.01; low certainty). Treatment: may shorten colds (MD -2.37 days, -4.21 to -0.53; I2 = 97%; 8 studies; low certainty). Non-serious adverse events probably increase with treatment (RR 1.34, 1.15-1.55; moderate certainty); no treatment study reported serious adverse events (abstract, PMID 38719213). 10.1002/14651858.CD014914.pub2
Karsch-Völk, M., Barrett, B., Kiefer, D., Bauer, R., Ardjomand-Woelkart, K., & Linde, K. (2014). Echinacea for preventing and treating the common cold. Cochrane Database of Systematic Reviews, (2), CD000530. 24 trials / 4,631 participants: at most a possible weak prevention benefit of questionable clinical relevance; no reliable treatment effect. 10.1002/14651858.CD000530.pub3
Tiralongo, E., Wee, S. S., & Lea, R. A. (2016). Elderberry supplementation reduces cold duration and symptoms in air-travellers: a randomized, double-blind placebo-controlled clinical trial. Nutrients, 8(4), 182. In 312 air travelers the reduction in cold incidence was not statistically significant (RR 0.69, low certainty); elderberry reduced cold duration and symptom score among those who did develop colds. 10.3390/nu8040182
Macknin, M., Wolski, K., Negrey, J., & Mallen, S. (2020). Elderberry extract outpatient influenza treatment for emergency room patients ages 5 and above: a randomized, double-blind, placebo-controlled trial. Journal of General Internal Medicine, 35(11), 3271-3277. Elderberry extract did not shorten influenza symptom duration versus placebo, contradicting earlier smaller trials. 10.1007/s11606-020-06170-w
Lissiman, E., Bhasale, A. L., & Cohen, M. (2014). Garlic for the common cold. Cochrane Database of Systematic Reviews, (11), CD006206. Of 8 studies only 1 was eligible (146 participants); it suggested fewer colds with daily garlic, but the evidence was judged insufficient to draw firm conclusions. The one trial randomised 146 people to a daily garlic supplement (180 mg allicin) or placebo for 12 weeks: 24 colds in the garlic group versus 65 with placebo (P < 0.001), 111 versus 366 days of illness, and similar days to recovery per cold (4.63 versus 5.63) (abstract, PMID 25386977). 10.1002/14651858.CD006206.pub4
Advisory Committee on Immunization Practices. (2025). Recommended Adult Immunization Schedule for Ages 19 Years or Older, United States, 2025. Centers for Disease Control and Prevention / MMWR. High-dose/adjuvanted/recombinant influenza preferred at 65+; pneumococcal at 50+; RZV shingles at 50+; RSV at 75+ (60-74 with risk factors); COVID-19 boosters for older adults. www.cdc.gov/vaccines/hcp/imz-schedules/adult-age.html
Centers for Disease Control and Prevention. (2024). Influenza Antiviral Medications: Summary for Clinicians. CDC. Antiviral treatment (e.g., oseltamivir) is recommended as early as possible, ideally within 48 hours of onset, for patients who are hospitalized, have severe/progressive illness, or are at higher risk of complications; clinical benefit is greatest with early treatment. www.cdc.gov/flu/hcp/antivirals/summary-clinicians.html
Kenealy, T., & Arroll, B. (2013). Antibiotics for the common cold and acute purulent rhinitis. Cochrane Database of Systematic Reviews, (6), CD000247. No evidence of benefit from antibiotics for the common cold; antibiotics caused significant adverse effects (about 1 in 10, mainly gastrointestinal). Routine use is not recommended. 10.1002/14651858.CD000247.pub3

Reading room/ConditionsReader