How to protect the nearly 1 in 15 adults with a weakened immune system

Key takeaways

• A study published in 2024 found that about 6.6% of U.S. adults, almost 1 in 15, had a weakened immune system in 2021, up from 2.7% in 2013¹².
• The “neutropenic diet,” a restrictive eating plan for cancer patients with low white blood cell counts, didn’t prevent infections in pooled trials; researchers recommend safe food handling instead⁴⁵.
• CDC advises that pregnant women and people with weakened immunity leave litter-box duty to someone else when they can, and it doesn’t recommend reptiles in their homes⁹.
• A 2026 infectious disease guideline says close contacts of someone severely immunosuppressed should get an inactivated flu shot rather than the live nasal vaccine, and CDC’s textbook gives similar advice¹¹¹².
• During chemotherapy, CDC says a temperature of 100.4°F (38°C) or higher calls for a phone call to your doctor right away, even in the middle of the night¹³.

Cancer patients whose infection-fighting white blood cells run low have commonly been put on a “neutropenic diet,” a restrictive eating plan meant to keep germs off the plate⁴. The idea makes intuitive sense. The cost is real, too: researchers link these diets with lower patient satisfaction and possibly malnutrition⁴.

When researchers pooled the trials, the diet didn’t deliver. Across five randomized trials with 388 patients, 53.7% of people on the restricted diet got infections, compared with 50% of those eating freely⁴. By our math, that’s about 4 more infections per 100 patients on the diet, a gap small enough to be chance⁴.

A larger 2019 analysis by Mayo Clinic researchers found the same. Among stem cell transplant patients, the restricted diet came with a slightly higher infection risk⁵. Their conclusion: no evidence supports the diet, and patients should keep following FDA’s safe food-handling guidance⁵.

That’s the larger story in small form. Some old rules don’t hold up, while plainer habits, like cooking sprouts and letting someone else scoop the litter box, do.

The group that needs protecting is growing. This guide covers who is at higher risk, which precautions have evidence, and how families can help without turning a home into a hospital.

Who’s at higher risk, and why the count keeps rising

A weakened immune system means the body’s defenses against infection are turned down, by a disease or by treatment. CDC’s list includes people with HIV or cancers such as leukemia, and people receiving chemotherapy, radiation, long-term steroids or other drugs that suppress the immune system³.

Organ transplant recipients taking anti-rejection drugs and people born with immune defects belong on the list too³.

In 2013 survey data, 2.7% of U.S. adults had a weakened immune system, according to CDC researcher Rafael Harpaz and colleagues, who analyzed 34,426 adults¹. Rates were highest in women and rose with age, peaking at 50 to 59¹.

Using 2021 data, a study published in JAMA in March 2024 found about 6.6%, almost 2.5 times as high². Our caution: we couldn’t open the JAMA paper to compare how each study defined a weakened immune system, so some of that jump may reflect method.

The newer study’s co-author, Melissa Martinson of the University of Washington, said the data can’t explain the jump for certain². She pointed to possible reasons: more immune-suppressing drugs prescribed for autoimmune disease, and more people learning their status during the pandemic².

Other conditions raise infection risk too. In diabetes, high blood sugar hampers the white blood cells that fight infection, a federal food safety booklet notes⁶. Age and pregnancy change immunity as well, as our article on immunity across the lifespan explains.

For people with weakened defenses, CDC says, recovery can take longer, and they may spread a virus for longer³. That matters for everyone in the household.

Food: safe handling beat the old food bans

During chemotherapy, infection-fighting white blood cells can drop, a condition called neutropenia. CDC says counts usually bottom out 7 to 12 days after each dose and can stay low for up to a week¹³.

Germs in food are a real threat during that time. The U.S. Department of Agriculture (USDA) and FDA say cancer patients and transplant recipients who get food poisoning are more likely to be sick longer, land in the hospital or die⁶. People with autoimmune disease taking two or more drugs for it face higher risk, too⁶.

Their booklet, last revised in March 2025, doesn’t ask anyone to give up fresh food. It names the riskiest versions of certain foods and the safer choice for each⁶:

• Sprouts. Raw alfalfa or bean sprouts can carry E. coli or Salmonella. Eat them cooked.
• Deli meats and hot dogs. Reheat until steaming hot, or 165°F.
• Soft cheeses. Choose ones labeled “made from pasteurized milk.” Skip raw-milk feta, Brie, Camembert and queso fresco.
• Smoked fish. Refrigerated smoked seafood can carry Listeria. Choose canned or shelf-stable, or heat it to 165°F.
• Milk, juice and cider. Choose pasteurized, or bring unpasteurized juice to a rolling boil for at least 1 minute.
• Eggs. Cook until yolks are firm, or 160°F. Skip raw cookie dough, homemade eggnog and homemade Caesar dressing.
• Meat and seafood. Skip raw or undercooked meat, sushi, sashimi and ceviche; cook to safe temperatures.
• Produce. Wash it, or cook it.

For readers who value fresh, whole food, that’s good news: the evidence supports keeping produce on the plate, washed⁵⁶. A food thermometer turns the numbers above into practice, and CDC recommends one for cooking meat safely¹⁰.

If someone gets sick after eating, the booklet says to contact a physician right away⁶. Save the food wrapped, labeled “DANGER” and frozen, and keep the packaging⁶.

Probiotics and herbs: “natural” doesn’t mean safe for every body

Many people take probiotics during treatment, hoping to protect their gut. In very sick patients, those live microbes can end up in the blood.

At Boston Children’s Hospital, 6 of 522 intensive care patients given a Lactobacillus probiotic developed bloodstream infections with it⁷. Among 21,652 patients who didn’t get the probiotic, only 2 had that kind of infection⁷. Genetic testing showed the blood bacteria matched the capsules⁷.

One detail stands out. Nearly all six had a central line, an IV tube into a large vein, and the researchers noted they lacked typical risk factors such as severe immune suppression⁷. Our reading: being very sick may matter as much as immune status. Our article on matching probiotics to the job covers this study and the special case of premature babies.

Herbs raise a different issue: interactions. NCCIH, the federal agency that studies complementary health, says there are theoretical reasons to suspect echinacea could interact with immune-suppressing drugs⁸. It advises talking with a provider before using echinacea with any medicine⁸.

An herb sold to rouse the immune system sits awkwardly beside a drug meant to calm it. Our echinacea article covers its other cautions, and a later article in this series covers supplements and autoimmune conditions.

Never stop or change a prescribed medicine to make room for a supplement. Bring every bottle to your care team.

Pets and gardens can stay, with a few rules

Animals and gardens are good for the spirit, and CDC’s advice keeps both in the picture.

Cats. If you’re pregnant or your immunity is weakened, CDC’s advice is to hand the litter box to another household member whenever possible⁹. When that isn’t an option, scoop with disposable gloves on, scrub your hands when you finish, and empty the box every day⁹. Cat germs include the parasite behind toxoplasmosis and the bacteria behind cat scratch disease and some stomach bugs⁹. By CDC’s estimate, somewhere between a fifth and four-fifths of cat bites and scratches that get reported turn into infections⁹.

Reptiles and amphibians. CDC says they aren’t recommended for homes with people who have weakened immune systems, young children or adults 65 and older⁹. Even healthy-looking ones can carry Salmonella⁹. If you have one, keep it and its gear out of the kitchen, and wash hands after any contact⁹.

Gardens. Toxoplasma, a parasite that infects more than 40 million Americans, can be picked up from soil or sand contaminated by cat feces¹⁰. CDC says it can cause severe illness in people with HIV, some chemotherapy patients and recent transplant recipients¹⁰. Wear gloves when gardening or touching soil or sand, and wash produce¹⁰.

Vaccines for the patient, and the people around them

The Infectious Diseases Society of America (IDSA) updated its guideline on seasonal vaccines for immunocompromised people on September 2, 2026¹². It strongly recommends flu, COVID-19 and RSV vaccines for this group¹².

IDSA notes that responses can be weaker, especially after transplants or drugs that deplete B cells, the cells that make antibodies¹². Timing helps: it advises vaccinating at least two weeks before new immune-suppressing treatment or chemotherapy when possible¹². It also says high-dose or adjuvanted flu vaccines produce a more strong response¹².

Live vaccines are different. CDC’s Pink Book, its textbook on vaccine-preventable diseases, says they usually shouldn’t be given to severely immunosuppressed people¹¹.

Because a patient’s own response may be weaker, the people nearby matter. Vaccinating household members to shield a vulnerable person is sometimes called cocooning. The Pink Book calls it critical that healthy household contacts of immunosuppressed people be vaccinated, and says most routine vaccines, even live ones, can be given to them¹¹.

There are a few exceptions:

• Nasal flu vaccine. IDSA says close contacts of severely immunosuppressed people, such as recent stem cell transplant recipients, should avoid the live nasal vaccine and get an inactivated flu shot instead¹².
• Chickenpox vaccine. If a rash appears after it, the Pink Book advises avoiding direct contact with an immunocompromised household member until the rash clears¹¹.

This describes what these groups advise; it isn’t a decision for anyone. Each person makes their own vaccine choices. Our vaccine schedule article covers who recommends what, and our companion article explains what happens in the body after a shot.

Speed matters: fever, testing and early treatment

For someone getting chemotherapy, a fever is an alarm. If your temperature reaches 100.4°F (38°C) or higher, CDC says to phone your doctor right away¹³. A fever can be the only clue to an infection, and during chemotherapy an infection can turn deadly¹³. CDC calls a possible infection an emergency and says not to wait until morning¹³.

If you end up at an emergency room, CDC suggests telling the person at the front desk that you’re in chemotherapy, so staff know to see you fast¹³. About 1 in 5 hospital stays for sepsis, the body’s extreme reaction to infection, are related to cancer¹⁴.

For flu and COVID-19, speed also matters. CDC’s December 2025 guidance for clinicians says antiviral treatment should start as soon as possible for higher-risk people, a group that includes those with weakened immunity¹⁵. Flu antivirals work best within two days of symptoms, but CDC still recommends them later for higher-risk patients¹⁵. COVID-19 antivirals are given within five to seven days¹⁵.

A positive test isn’t required to start treatment, CDC says¹⁵. It also suggests planning with a provider how you’d get treatment before you need it¹⁵.

When to get help

• During chemotherapy: call your doctor right away for a temperature of 100.4°F (38°C) or higher¹³.
• Signs of infection in someone with cancer: fever, chills or sweats; changes in urination or thinking; diarrhea or vomiting; a new mouth sore or painful swallowing; shortness of breath; a stiff neck¹⁴.
• Possible sepsis: a fast heartbeat or low blood pressure, shivering or feeling very cold, confusion, extreme pain, clammy skin or shortness of breath. CDC calls sepsis an emergency; get treated immediately or go to the emergency room¹⁴. Our article on inflammation and sepsis covers when to call 911.
• Flu symptoms in a higher-risk person: contact a doctor early¹⁵. For emergency warning signs, CDC says to go to the emergency room¹⁵.
• Food poisoning symptoms: contact a physician right away⁶.

The household protection plan

Caring for a vulnerable person is one of the oldest forms of community health. Martinson notes that without community protections, some people with weakened immune systems feel isolated². A household plan lets them stay connected.

This plan is our own summary of the sources cited.

Area	What to do	Why	Source
Kitchen	Cook sprouts; reheat deli meats to 165°F; choose pasteurized milk, juice and soft cheeses; use a food thermometer	Foodborne illness hits harder after chemo or a transplant	610
Diet	Keep fresh produce; wash it	Restrictive diets didn’t prevent infections	456
Cats	Someone else changes the litter, daily; gloves if not	Toxoplasmosis and other cat-borne germs	910
Reptiles	Not recommended in the home; keep out of the kitchen if present	Salmonella, even in healthy animals	9
Garden	Gloves for soil and sand; wash produce	Toxoplasma lives in soil cats use	10
Visitors and sick family	Wash hands often; sick people stay home and avoid contact	Germs spread by hands and close contact	1315
Vaccines	Ask the care team about timing; household members can ask about inactivated flu shots	Patient responses may be weaker	1112
Supplements	Show every product to the care team; don’t change medicines on your own	Probiotic infections in very sick patients; herb-drug interactions	78
Symptoms	Keep a thermometer and after-hours numbers handy; plan treatment access ahead	Fever may be the only sign of infection; antivirals work best early	1315

Everyday layers, from handwashing to ventilation and staying home when sick, are covered in two companion articles in this series on hygiene and on preventing respiratory infections.

How strong is the evidence?

Claim	Evidence level	Basis
A neutropenic diet prevents infections during cancer treatment	Evidence says no, or unsafe	Two meta-analyses of randomized trials; slightly more infections in transplant patients45
Foodborne germs hit people with weakened immunity harder	Strong evidence	Federal food safety guidance and data6
Probiotics can cause bloodstream infections in very sick patients	Strong evidence	Hospital data with genetic matching7
Probiotics are risky for immunocompromised people at home	Not enough evidence	The hospital study involved intensive care patients with central lines7
Echinacea interacts with immune-suppressing drugs	Not enough evidence	NCCIH calls the concern theoretical8
Immunocompromised people respond less well to vaccines	Strong evidence	IDSA 2026 guideline review12
Early antivirals help higher-risk people with flu or COVID-19	Strong evidence	CDC clinical guidance15

Questions readers ask

The bottom line

The people who need extra protection are a growing share of the country, and their risks are real: a fever during chemotherapy can be the only sign of a dangerous infection. The best-supported precautions are practical, such as cooking risky foods, letting someone else handle the litter, keeping soil off hands and acting fast on fever.

Some well-meant rules, like the neutropenic diet, didn’t hold up. Families can help most by learning the plan, staying home when sick and talking with the care team about vaccines and supplements.

Keep exploring: exercise and immune system.

Related reading

• Babies borrow their mothers’ antibodies, and older adults’ infections may bring no fever
• Inflammation isn’t always infection, and sepsis is the infection that can’t wait
• Probiotics work for a few jobs, so match the strain or skip the bottle
• How to read competing vaccine schedules now that the advice has split
• A sore arm after a shot is your immune system at work, and some of it you can control

Sources

1. Healio, Infectious Disease News. “Nearly 3% of US Adult Population Immunosuppressed” (on Harpaz R, Dahl RM, Dooling KL, “Prevalence of Immunosuppression Among US Adults, 2013,” JAMA, 2016;316(23):2547). December 2016. healio.com
2. University of Washington News. “Q&A: Melissa Martinson on Immunosuppression” (on Martinson ML, Lapham J, “Prevalence of Immunosuppression Among US Adults,” JAMA, March 2024). March 13, 2024. washington.edu
3. Centers for Disease Control and Prevention. “People at Increased Risk for Flu Complications,” September 11, 2024, and “People at Increased Risk for Severe Respiratory Illnesses,” August 18, 2025. cdc.gov and cdc.gov
4. Ball S, Brown TJ, Das A, Khera R, Khanna S, Gupta A. “Effect of Neutropenic Diet on Infection Rates in Cancer Patients With Neutropenia: A Meta-analysis of Randomized Controlled Trials.” American Journal of Clinical Oncology, 2019;42(3):270–274 (University of Minnesota record). experts.umn.edu
5. Sonbol MB, Jain T, Firwana B, et al. “Neutropenic Diets to Prevent Cancer Infections: Updated Systematic Review and Meta-Analysis.” BMJ Supportive & Palliative Care, 2019;9(4):425–433 (Mayo Clinic record). mayoclinic.elsevierpure.com
6. U.S. Department of Agriculture Food Safety and Inspection Service and U.S. Food and Drug Administration. “Food Safety for Older Adults and People with Cancer, Diabetes, HIV/AIDS, Organ Transplants, and Autoimmune Diseases.” Revised July 2019, slightly revised March 2025. fsis.usda.gov
7. Yelin I, Flett KB, Merakou C, et al. “Genomic and Epidemiological Evidence of Bacterial Transmission From Probiotic Capsule to Blood in ICU Patients.” Nature Medicine, 2019;25:1728–1732 (Technion author copy). pubmed.ncbi.nlm.nih.gov
8. National Center for Complementary and Integrative Health. “Echinacea.” NIH, updated November 2024. nccih.nih.gov
9. Centers for Disease Control and Prevention, Healthy Pets. “Cats,” June 15, 2026, and “Reptiles and Amphibians,” January 13, 2026. cdc.gov and cdc.gov
10. Centers for Disease Control and Prevention. “About Toxoplasmosis.” CDC, March 11, 2025. cdc.gov
11. Centers for Disease Control and Prevention. “Chapter 2: General Best Practice Guidance for Immunization.” Pink Book, updated April 22, 2024. cdc.gov
12. Infectious Diseases Society of America. “IDSA 2026 Guidelines on the Use of Vaccines for the Prevention of Seasonal COVID-19, Influenza, and RSV Infections in Immunocompromised Patients.” IDSA, updated September 2, 2026. idsociety.org
13. Centers for Disease Control and Prevention. “Watch Out for Fever,” November 14, 2024; “Information for Patients Who Are Getting Chemotherapy,” August 25, 2025; and “Preventing Infections in Cancer Patients,” reviewed November 14, 2024. cdc.gov, cdc.gov and cdc.gov
14. Centers for Disease Control and Prevention. “For Cancer Patients and Their Caregivers: Cancer, Infection, and Sepsis.” Fact sheet, undated. cdc.gov
15. Centers for Disease Control and Prevention. “Clinical Guidance for Outpatients With Acute Respiratory Illness at Higher Risk of Severe COVID-19 and/or Influenza,” December 19, 2025; “Treatment of Respiratory Viruses,” August 18, 2025; and “Flu: What To Do If You Get Sick,” August 30, 2024. cdc.gov, cdc.gov and cdc.gov