Preventing and spotting infections

Why infections are more likely during treatment, how to lower your risk, and when a fever is an emergency.

Rosa: Hi, I'm Rosa. Some treatments make it harder for your body to fight germs. Let's go over how to lower your risk, and when a fever means you should call right away.

Many cancer treatments lower the number of white blood cells. These cells fight germs. When they are low, even a small infection can become serious very fast.

White blood cells often drop to their lowest point about 1 to 2 weeks after chemotherapy. Some treatments, like stem cell transplant, CAR T-cell therapy, and some leukemia treatments, lower your defenses for longer. Ask your care team when your risk is highest.

A fever may be the only sign of infection when your white cells are low. You may not have redness, pus, or a cough. That is why a fever during treatment needs a call to your care team right away. This is true even at night or on a weekend.

Do not take fever medicine (like acetaminophen or ibuprofen) before you call. Only take it first if your team told you to. It can hide a fever your team needs to know about.

Your care team may give you medicines to prevent infections. These can include antibiotics, antiviral pills to prevent shingles, or antifungal medicines. Some people get a shot that helps the body make white blood cells. Some people get a hepatitis B blood test before treatment. Some cancer drugs can wake up an old infection.

Vaccines can protect you, but timing matters. Flu, COVID-19, pneumonia, and shingles vaccines that are not live are usually safe. Live vaccines are usually not given during treatment. Ask your care team before getting any vaccine. Tell anyone giving you a vaccine that you are getting cancer treatment.

Fever is an emergency

Call your care team right away for a temperature of 100.4°F (38°C) or higher. Use a different number if your team gives you one. If you cannot reach them, go to the emergency room and say you are getting chemotherapy. If it feels very bad, call 911.

Other warning signs

Chills or shaking, sweating, a new cough, or trouble breathing. Burning when you pee, a sore throat, mouth sores, or diarrhea. Redness, swelling, or pain around a port, catheter, or wound.

Wash your hands

Handwashing is the best way to prevent infection. Wash before eating, after using the bathroom, and after being out in public. Ask visitors to wash too.

Avoid sick people

Stay away from people with colds, flu, or other illness. Consider a mask in crowds or during flu season.

Food safety

Cook meat, fish, and eggs all the way through. Wash fruits and vegetables. Avoid unpasteurized milk, juice, and cheese. Ask if you need a special diet.

Care for your mouth and skin

Brush gently with a soft toothbrush. Keep skin clean and moisturized. Clean cuts right away. Wear gloves when gardening.

Pets

You can usually keep your pets. Wash your hands after touching them. Let someone else clean litter boxes, bird cages, and fish tanks.

Myth: a fever can wait until morning

Fact: when your white cells are low, infection can get worse within hours. Call right away. Early antibiotics can be lifesaving.

• Keep a working thermometer at home and know how to use it.
• Save your care team's after-hours phone number in your phone.
• Ask when your white blood cells are expected to be lowest.
• Ask which vaccines you should get, and when.
• Ask if you need medicines to prevent infection.
• Carry a card or note saying you are on cancer treatment.

Last updated: 2026-10-27. Medically reviewed by Veli Bakalov, MD, medical oncologist (October 2026). General education from Step by Step, the patient portal of HemeOncBuddy.