What is an infection?
During leukemia treatment you will hear the word “infection” very often. But what exactly is an infection, where do germs come from and how do they get into the body, how do doctors find them, and why can even an ordinary germ become dangerous when immunity is weak? This topic lays the groundwork for the following topics on bacterial, viral and fungal infections.
01Short answer
02Simple explanation
We are surrounded everywhere by living things too small to see: germs, or microbes. Most of them are harmless, and some are even helpful; for example, gut bacteria help us digest food. Only a small share of them can cause disease.
An infection is when a germ gets into the body, multiplies there and damages tissues. A germ simply living on the skin or in the gut is not yet an infection; this is called “colonization”. The trouble starts when a germ gets past a protective barrier and reaches a place where it should not be, such as the blood, the lungs or the urinary tract.
When the immune system responds to a germ, we feel the signs of infection: fever, chills, pain, redness, swelling, cough, diarrhea. Much of this is produced by white blood cells. When they are few, the body still fights the germ, but the “alarm signal” comes out weak.
In leukemia, many of the germs that cause infections do not come from outside but from the person’s own body. When chemotherapy damages the lining of the mouth and gut, or a catheter opens a “door” in the skin, ordinary bacteria and fungi can get inside.
The good news is that most infections can be treated when they are found in time. So the main rule is simple: if you have a fever or signs of infection, do not wait; call your team straight away.
03Scientific explanation
Colonization and infection. Trillions of germs, the microbiota, live on the skin and in the nose, mouth and gut of a healthy person. They are usually harmless and even stop harmful germs from settling in. Infection happens when a germ enters tissues, multiplies and triggers a response from the body. Infections that, in people with weak immunity, are caused by germs that do not harm healthy people are called opportunistic infections.
Where do the signs come from? Immune cells that recognise a germ release inflammatory substances (cytokines): these widen blood vessels, call white cells to the site and “reset” the temperature centre in the brain. Redness, swelling and pus are largely the result of neutrophils at work. The IDSA guideline stresses that during neutropenia signs of inflammation may be minimal and a fever is often the only early sign of infection.
How is an infection looked for? With a fever, at least two sets of blood cultures are usually taken (if there is a catheter, from the catheter and from a vein in the arm), and urine, sputum, stool and swabs are tested if needed. Culture results and the germ’s sensitivity to medicines usually take 1–3 days. Viruses are most often looked for in a nose or throat swab by PCR, and some fungi with blood antigen tests and CT scans. In many fever episodes no specific germ is ever found; this does not mean there is no infection.
Types of risk. The ASCO/IDSA guideline judges risk by the type and length of immune weakness: neutrophils staying below 0.1×10⁹/L for more than 7 days count as high risk, and in that case preventive antibiotics and antifungal medicines are recommended. Weakened T lymphocytes (for example with long-term steroids or a transplant) raise the tendency to viral infections and Pneumocystis, while low antibody levels raise the tendency to repeated bacterial infections. According to WHO, about one in six laboratory-confirmed bacterial infections worldwide were resistant to antibiotics in 2023; this is why using antibiotics correctly matters for everyone.
04What happens in the body?
When should you seek urgent help?
During treatment, call your hematology team’s 24-hour number or emergency services straight away if your temperature is 38 °C or higher (if your team gave you a different threshold, follow it), or if you have chills, sudden weakness, confusion, breathlessness, burning when passing urine, diarrhea, mouth sores or redness around your catheter. Do not take fever-reducing medicine before measuring your temperature; it can hide the signs.
Where do germs come from?
Some come from outside: the coughs of sick people, unwashed hands, food that was not prepared safely, soil and building dust. But in people with leukemia, the source of many infections is their own germs: bacteria from the skin, mouth and gut, and the fungus Candida. So you do not need to be in contact with anyone to get an infection.
How do germs get inside?
The body’s main barriers are the skin and the mucous membranes. Chemotherapy can cause sores in the mouth and gut (mucositis), catheters and needle sites open a “door” in the skin, and a weaker cough reflex leaves the lungs vulnerable. Mouth care, catheter care and cleaning skin injuries promptly help close these “doors”.
How are infections found?
The doctor examines you, takes blood tests and, to find the germ itself, sends blood, urine and other samples to the laboratory to be cultured. Sometimes a chest X-ray or CT scan is needed. Waiting for results can be dangerous, so antibiotics are often started as soon as samples are taken and then adjusted when the results come back.
Is an infection contagious?
Some infections (flu, COVID-19, chickenpox) pass from person to person, while others, such as a blood infection caused by your own gut bacteria or the mould Aspergillus, are not contagious. Leukemia itself is not contagious either. Hand hygiene, vaccinations for your loved ones and keeping their distance when they are ill all protect you.
Questions you can ask your doctor
How high is my risk of infection now, and when will it be highest? Which signs mean I should call straight away, and which number should I use? Do I need preventive medicines? How should I look after my catheter? What precautions should my family take? If an infection is found, will it be treated at home or in hospital?
05How does an infection develop?
Simplified: from germ to signs.
06Main types of germs (simplified)
| Type | Examples | How is it treated? |
|---|---|---|
| Bacteria | Staphylococci, streptococci, E. coli, Pseudomonas | Antibiotics |
| Viruses | Flu, COVID-19, RSV, herpes viruses, CMV | Antiviral medicines for some; vaccines protect |
| Fungi: yeasts | Candida (white coating in the mouth, blood infection) | Antifungal medicines |
| Fungi: moulds | Aspergillus, mucormycetes | Antifungal medicines, sometimes surgery |
| Pneumocystis | Lung infection (PCP) | A special antibiotic combination; prevention is possible |
| Parasites | Toxoplasma and others (rare) | Antiparasitic medicines |
* The table is for general understanding. Your hematology team decides which germ is suspected and what treatment is needed.
07Key terms
- Infection
- A germ getting into the body, multiplying and causing harm.
- Germ (microbe)
- A living thing too small to see: a bacterium, virus, fungus or parasite.
- Pathogen
- A germ that can cause disease.
- Colonization
- A germ living on a body surface without causing disease.
- Microbiota
- The normal germs living on the skin and in the mouth and gut.
- Opportunistic infection
- An infection by a germ that usually only causes disease when immunity is weak.
- Inflammation
- The body’s response to injury or germs: redness, swelling, pain, heat.
- Culture
- Growing a germ from a sample on a special medium in the laboratory.
- Empirical treatment
- Treatment aimed at the most likely causes, started before the germ is known.
- Antimicrobial resistance
- Germs no longer responding to medicines.
08Key points
- An infection is a germ getting into the body, multiplying and causing harm.
- Most germs are harmless; with weak immunity, our “own” germs can become dangerous.
- In leukemia, infections start more easily and can spread quickly.
- During neutropenia signs are faint; a fever is often the only signal.
- Infections are looked for with cultures, PCR and imaging.
- Treatment often starts without waiting for results.
- Antibiotics do not work against viruses.
- With a fever, call your team straight away.
09Frequently asked questions
Does every fever mean an infection?
No. A fever can also be caused by some medicines, blood transfusions or the leukemia itself. But during treatment every fever is treated as an infection until proven otherwise, because delay can be dangerous.
How can germs from my own body cause an infection?
When you are healthy, the skin, the mucous membranes and white blood cells keep these germs in their place. When chemotherapy weakens these barriers, normally harmless bacteria and fungi can pass into the blood or tissues. This does not mean your hygiene is poor.
Why are antibiotics started before the results come back?
During neutropenia a bacterial infection can become serious within hours. So doctors first start a broad-spectrum antibiotic against the most likely germs, and then adjust treatment when the results arrive.
No germ was found. Does that mean there was no infection?
Not necessarily. In many fever episodes no germ is found: there may be very few of them, or the antibiotic may already have worked. The doctor decides whether to continue treatment based on your overall condition.
Is it worth taking antibiotics for a cold?
Antibiotics do not work against viral infections such as colds and flu, but they can cause side effects and resistance. During leukemia treatment, do not start any medicine on your own; call your team first.
Can my family catch my infection?
It depends on the germ. Flu, COVID-19 and some gut infections are contagious, while most blood infections and fungal infections are not. Your team will tell you which precautions are needed.
10Questions for your doctor
Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.
How high is my infection risk right now?
Which signs of infection mean I should call straight away?
Until when should I take my preventive medicines?
Will an infection delay my treatment?
12Sources
- National Cancer Institute (NCI). Infection and Neutropenia during Cancer Treatment
- MedlinePlus (U.S. National Library of Medicine). Infections
- MedlinePlus (U.S. National Library of Medicine). Germs and Hygiene
- MedlinePlus (U.S. National Library of Medicine). Bacterial Infections
- American Cancer Society. Infections in People with Cancer
- American Cancer Society. Why Are People with Cancer More Likely to Get Infections?
- Centers for Disease Control and Prevention (CDC). Information for Patients Who Are Getting Chemotherapy
- Centers for Disease Control and Prevention (CDC). Healthy Habits: Antibiotic Do’s and Don’ts
- Cancer Research UK. Sepsis, infection, and cancer
- Blood Cancer UK. Blood cancer and infection
- World Health Organization (WHO). Antimicrobial resistance (fact sheet)
- Journal of Clinical Oncology (ASCO and IDSA), 2018. Antimicrobial Prophylaxis for Adult Patients With Cancer-Related Immunosuppression: ASCO and IDSA Clinical Practice Guideline Update
- Clinical Infectious Diseases (IDSA), 2011. Clinical practice guideline for the use of antimicrobial agents in neutropenic patients with cancer: 2010 update by the Infectious Diseases Society of America