The short answer
Cancer treatments lower white blood cell counts, which makes infection easier to catch and harder to fight. NCI advises staying away from people who are unwell and from crowds, washing hands frequently with soap and warm water, and calling the care team at any sign of infection.
NCI says cancer treatments lower white blood cell counts, increasing vulnerability to infection.
Its advice includes staying away from ill individuals and from crowds.
Frequent hand washing with soap and warm water is the core habit, especially before eating.
NCI also advises avoiding people who have recently had live vaccines such as chickenpox, polio or measles.
On this page
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The whole article, with every detail and caution.
This page is for people having chemotherapy who want to lower their chance of catching an infection, and for the people around them. It covers warning signs, NCI's prevention habits, and practical ways to keep a social life.
When to get help sooner
Call 911 if you become severely unwell — struggling to breathe, unable to stay awake, confused, or with a stiff neck and violent shaking chills. NCI states that infections during cancer treatment can be life threatening and require urgent medical attention, and a rapid collapse is an emergency rather than a clinic call. Do not drive yourself.
A fever of 100.4 °F (38 °C) or higher during chemotherapy is a medical emergency, CDC says. Call your doctor or your team's 24-hour number immediately. If you go to the emergency room, tell staff you are having chemotherapy.
Call your care team right away, at any hour, for chills or sweats, a new cough, sore throat or new mouth sore, diarrhea, a stiff neck, pain when you urinate, or swelling, soreness or redness where a catheter enters the body (CDC). NCI's list of infection signs also includes ear, head or sinus pain, skin rash, and urine that is bloody or cloudy; call right away for those too. NCI says infections during cancer treatment can be life threatening and need urgent medical attention.
One more instruction from NCI matters here: avoid taking medicine to bring a fever down without consulting your doctor first, because it may mask a serious problem.
Why a crowd is different for you now
The reason is mechanical rather than mysterious. NCI explains that cancer treatments lower white blood cell counts, increasing infection vulnerability. White cells are the part of the blood that fights germs off. With fewer of them, an ordinary exposure becomes a bigger event.
A crowded indoor room simply increases the number of exposures per hour. More people, less air, more shared surfaces. Nothing about the germs has changed. Your ability to shrug them off has.
The aim is fewer chances for a germ to reach you, not a sealed life.
What NCI actually advises
Its infection prevention guidance is a short set of habits.
- Hands. Wash frequently with soap and warm water, particularly before eating, and encourage other people to do the same.
- People. Stay away from those who are ill, and away from crowds.
- Recent live vaccines. Avoid people who have recently received live vaccines such as chickenpox, polio or measles vaccines.
- Catheters. If you have a catheter, keep the area around it clean and dry.
- Mouth. Clean your teeth regularly and inspect your mouth daily for sores or signs of infection.
- Food. Cook meat, fish and eggs thoroughly, keep food at proper temperatures, and either eat fruit and vegetables you can peel or wash raw produce thoroughly.
NCI also notes that stress, poor nutrition and insufficient sleep further weaken immune function. That makes rest and eating well as important as hand washing, not just nice extras.
Making it livable
The advice can read as a sentence of isolation. In practice most people find workable versions of their normal life.
- Shift the timing. The same supermarket at eight in the morning is a different place from the same supermarket on Saturday afternoon.
- Move it outdoors. A walk with a friend replaces a crowded café without canceling the friendship.
- Shrink the group. Two people in your kitchen is not the same risk as thirty in a hall.
- Ask people to be honest. A friend who says "I have a cold, shall we do next week?" is doing you an enormous favor.
- Use delivery and online ordering where you can, and save your outings for the things you actually want to do.
Say it out loud to the people around you
Family and friends often have no idea that a mild cold in them is a serious matter for you. Most will happily change plans if they know why, and will be mortified to learn afterwards that they should have stayed away.
Tell them directly. Ask them to:
- wash their hands when they arrive
- postpone if they are unwell
- be honest rather than polite
And ask your own team where your risk sits across your cycle. Counts move, and the caution that is essential one week may be relaxed the next.
Sources
Common questions
Does avoiding crowds mean staying indoors all the time?
NCI's advice is to stay away from ill individuals and crowds, not to stop living. Outdoor space, quiet times of day and smaller gatherings all reduce exposure without shutting you in.
Why is hand washing singled out so often?
NCI advises washing hands frequently with soap and warm water, particularly before eating, and encouraging others around you to do the same. It is the first habit NCI lists for preventing infection.
Why should I avoid someone who has just been vaccinated?
NCI advises avoiding people who recently received live vaccines, naming chickenpox, polio and measles vaccines. That is a specific and limited exception, not a reason to discourage vaccination in general.
How do I know if I have an infection?
NCI lists fever, chills, cough, sore throat, diarrhea, ear, head or sinus pain, stiff neck, skin rash, mouth sores, swelling or redness where a catheter enters the body, and urine that is bloody or cloudy or painful to pass. CDC says to call your doctor immediately for a temperature of 100.4 °F (38 °C) or higher during chemotherapy, and to call right away for chills, a new cough, sore throat or mouth sore, diarrhea, or redness at a port or catheter.
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Get urgent help
Fever of 100.4°F (38°C) or higher during chemo, severe pain, or trouble breathing: call your cancer care team right away, day or night. Call 911 for an emergency.
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Sources last checked: 2026-08-11 what this meansLast updated: 2026-09-29Next planned review: 2027-08-11
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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