Immunity and infection prevention

Viral infections

Viruses are much smaller than bacteria and can only multiply inside our own cells. In leukemia and during its treatment, both viruses that come from outside (flu, COVID-19, RSV) and viruses that “sleep” in the body for years (herpes, chickenpox and shingles, cytomegalovirus, hepatitis B) can cause more serious illness. This topic explains the types of viral infection, their signs, the tests used and the ways to stay protected.

Last updated: 3 October 2026~7 min read

A large translucent glowing cell on a dark navy background, with a few small spiky spherical virus particles approaching its surface. — **Figure 1.**Concept: a virus attaches to the surface of a cell and tries to get inside.

01Short answer

Viruses are germs that enter our cells and multiply inside them; antibiotics do not work against them. The main defence against viruses comes from T lymphocytes, NK cells and antibodies, so treatments that weaken lymphocytes (steroids, some chemotherapy drugs, transplants, CAR-T) raise the risk of viral infections. In people with leukemia there are two main groups: respiratory and gut viruses caught from others, and the reawakening of viruses hiding in the body (herpes viruses, hepatitis B). Viruses are mostly found with PCR tests. Some have antiviral medicines, others are prevented or regularly monitored; vaccines and hygiene protect. Your hematology team decides which measures you need.

02Simple explanation

A virus is a very small germ that cannot live on its own. It enters one of our cells, uses the cell’s “machinery” to make new viruses, and these spread to nearby cells. This is why antibiotics, which kill bacteria, do not work against viruses.

The main fighters against viruses are lymphocytes, one kind of white blood cell: T cells find and destroy infected cells, and B cells make antibodies. When leukemia and its treatment weaken lymphocytes, viruses multiply more easily.

Some viruses come from outside: flu, COVID-19, RSV and common cold viruses spread through coughs, sneezes and unwashed hands. In a healthy person these infections are mild, but with weak immunity they can spread to the lungs and cause pneumonia.

Other viruses stay in the body in a “sleeping” state from childhood, for example the herpes virus that causes cold sores, the chickenpox virus and cytomegalovirus. When immunity weakens, they can “wake up”: the chickenpox virus as shingles, and the herpes virus as painful mouth sores.

The good news is that many viral infections can be prevented or treated early. Preventive antiviral medicines, monitoring with blood tests, vaccines and simple hygiene rules lower the risk considerably.

03Scientific explanation

Reawakening herpes viruses. According to the 2022 guideline of the Infectious Diseases Working Party (AGIHO) of the German Society for Hematology and Medical Oncology, herpes simplex reactivation shows up especially as stomatitis (mouth inflammation) in patients with acute leukemia on intensive chemotherapy, while varicella zoster virus appears mainly as shingles, more often in lymphoma and myeloma. The ASCO/IDSA guideline recommends acyclovir-type prophylaxis for herpes simplex-seropositive patients having an allogeneic transplant or leukemia induction treatment. Shingles prophylaxis is considered with some medicines and after a transplant.

Cytomegalovirus (CMV). CMV is one of the most important infections after an allogeneic transplant; according to the ECIL 7 guidelines, it can also matter in patients treated with some newer drugs. After a transplant, CMV is usually measured regularly in the blood by PCR, and if the level rises, treatment starts before disease develops. In a 2017 NEJM trial in CMV-seropositive allogeneic transplant recipients, letermovir prophylaxis reduced clinically significant CMV infection by week 24 from 60.6% to 37.5%.

Hepatitis B reactivation. According to the 2020 ASCO opinion, every patient expecting systemic cancer treatment should be tested with three tests (HBsAg, anti-HBc and anti-HBs), but this should not delay treatment. People with chronic hepatitis B should receive antiviral prophylaxis throughout treatment and for at least 12 months afterwards. Those with past hepatitis B receiving high-risk treatment, such as anti-CD20 antibodies (for example rituximab) or a stem cell transplant, are also advised to have prophylaxis or very close monitoring.

Respiratory viruses. The European ECIL-10 recommendations (2026) cover infection control, PCR testing and, in some cases, considering a delay of chemotherapy or transplant for all respiratory viruses. For flu, a seasonal inactivated vaccine and early antiviral treatment are recommended, while routine antiviral prophylaxis is discouraged. RSV vaccines can be considered according to local approval. For many other viruses there is no specific treatment: supportive care, correcting low antibody levels and, where possible, reducing steroids are recommended.

04What happens in the body?

When should you seek urgent help?

Call your team straight away if you have a fever, breathlessness, fast breathing, chest pain or confusion, or if you develop a painful blistering rash on one side of the body, a rash near the eye, or mouth sores that make swallowing hard. If you have been in contact with someone with chickenpox or shingles, tell your team without waiting for symptoms, as preventive treatment may be needed.

Viruses from outside

Flu, COVID-19, RSV, parainfluenza, metapneumovirus, rhinovirus and adenovirus spread through coughs, sneezes and hands. An infection that starts with a runny nose, sore throat and cough can move down to the lower airways when immunity is weak. Gut viruses such as norovirus cause vomiting and diarrhea. Hand hygiene, keeping away from sick people and vaccinations for your family are the main protection.

When “sleeping” viruses wake up

Herpes simplex (cold sores, mouth ulcers), varicella zoster (a painful band of blisters on one side of the body), cytomegalovirus and Epstein–Barr virus can stay hidden for years. With some treatments, preventive medicines are given against them or the blood is checked regularly. It is important to take preventive medicines until the end of the course.

A fine glowing line resembling a translucent nerve fibre on a dark background; inside it a few dim points begin to glow again with a soft golden light. — **Figure 2.**Some viruses “sleep” for years and can wake up when immunity weakens.

How are viruses found?

For respiratory viruses, a nose and throat swab is taken and a PCR panel that checks several viruses at once is used. A swab can also be taken from herpes sores. CMV, hepatitis B and some other viruses are measured in the blood by PCR. If needed, a CT scan of the lungs and a bronchoscopy are done.

Treatment and prevention

There are antiviral medicines against herpes viruses, CMV, flu, COVID-19 and hepatitis B; they work better when started early. For many other viruses treatment is supportive: fluids, oxygen, rest and sometimes immunoglobulin. We have a separate topic on vaccines; your team decides which vaccine is possible and when.

Several glowing round lymphocytes surrounding an infected cell on a dark navy background, linked by thin teal bridges of light. — **Figure 3.**T lymphocytes are the main defenders that find and destroy infected cells.

Questions you can ask your doctor

Do I need herpes or shingles prophylaxis, and for how long? What did my hepatitis B test show? How will CMV be monitored after my transplant? What should I do if I get cold symptoms, and could my treatment be postponed? Which vaccines do my family and I need? Whom should I call if I have been in contact with chickenpox?

05How does a viral infection develop?

Simplified: from catching or reawakening to treatment.

1Catching or wakingA virus from outside, or one sleeping in the body.

2Multiplying in cellsThe virus enters cells and multiplies.

3SignsFever, cough, rash, sores, or no signs at all.

4Test and treatPCR, antiviral medicine or supportive care.

Antiviral medicinesPrevention and treatment for some viruses.

MonitoringCMV and hepatitis B tracked in the blood by PCR.

ProtectionVaccines, hygiene, distance.

06Important viruses in leukemia (simplified)

Virus	How it may show	What helps?
Herpes simplex	Cold sores, painful mouth ulcers	Antiviral prophylaxis and treatment
Varicella zoster	A painful band of blisters on one side of the body; chickenpox	Antiviral medicines; a vaccine in some cases
Cytomegalovirus (CMV)	Often no signs; fever, falling blood counts, pneumonia	PCR monitoring of the blood, prophylaxis or early treatment
Flu and COVID-19	Fever, cough, aching muscles	Vaccine, early antiviral treatment
RSV and other respiratory viruses	Runny nose, cough, wheeze, breathlessness	Hygiene, supportive care; sometimes a vaccine
Hepatitis B	Often no signs; rising liver tests	Testing before treatment, antiviral prophylaxis

* The table is for general understanding. Prevention and treatment depend on the type of therapy and the level of risk; your hematology team makes the decision.

07Key terms

Virus: A very small germ that can only multiply inside a living cell.
Reactivation: A virus hiding in the body becoming active again.
Latent infection: A virus “sleeping” in the body without causing signs.
Herpes viruses: A family of viruses including herpes simplex, varicella zoster, CMV and EBV.
Shingles: A painful blistering rash caused by the chickenpox virus waking up.
CMV: Cytomegalovirus; a herpes virus that matters especially after a transplant.
PCR: A sensitive test that finds and measures a virus’s genetic material.
Antiviral medicine: A medicine that stops a virus from multiplying.
Seropositive: Having antibodies to a virus in the blood, a sign of past infection.
Hepatitis B reactivation: Hidden hepatitis B virus multiplying again when immunity weakens.

08Key points

Viruses multiply inside cells; antibiotics do not work against them.
Treatments that weaken lymphocytes raise the risk of viral infections.
With weak immunity, respiratory viruses can spread to the lungs.
“Sleeping” viruses such as herpes, shingles and CMV can wake up.
A hepatitis B test before treatment matters.
Viruses are mostly found with PCR tests.
Take preventive antiviral medicines until the end of the course.
Vaccines and hygiene protect you and your family.

09Frequently asked questions

I have cold symptoms. Is this urgent too?

If you have a fever, breathlessness or weakness, yes: call straight away. Even with just a mild runny nose, tell your team: they can decide whether to test, how to protect your family and sometimes whether to postpone treatment.

Why am I given a medicine like acyclovir when I am not ill?

It is prevention: it stops herpes viruses hiding in the body from waking up during treatment. Take it for as long as your doctor says and do not stop it on your own.

My CMV test is positive. What does that mean?

If it refers to antibodies, it means you were infected with CMV at some point in the past, as most people are. After a transplant, the virus itself is tracked in the blood by PCR, and treatment starts if the level rises.

I have had hepatitis B in the past. Can I still have leukemia treatment?

Yes. Tests are done before treatment, antiviral prophylaxis is started if needed, and liver tests are monitored. This usually does not delay leukemia treatment.

My child brought a cold home from school. What should I do?

Wash hands often, sleep in separate rooms if possible, avoid sharing items and consider a mask. If you develop symptoms, tell your team. Your family’s flu and COVID-19 vaccinations protect you too.

Is shingles contagious?

The virus in shingles blisters can pass to people who have never had chickenpox or the vaccine and give them chickenpox. Keeping the blisters covered until they dry and avoiding contact with vulnerable people is advised.

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.

Which virus tests were done, and what are the results?
Are viruses like herpes or cytomegalovirus a danger for me?
Do I need antiviral prevention?
How should we behave if someone in the family has a cold?

My questions

12Sources

MedlinePlus (U.S. National Library of Medicine). Viral Infections
MedlinePlus (U.S. National Library of Medicine). Shingles
MedlinePlus (U.S. National Library of Medicine). Cytomegalovirus Infections
Centers for Disease Control and Prevention (CDC). About Shingles (Herpes Zoster)
Centers for Disease Control and Prevention (CDC). About RSV
Centers for Disease Control and Prevention (CDC). Hepatitis B Basics
World Health Organization (WHO). Hepatitis B (fact sheet)
National Cancer Institute (NCI). Infection and Neutropenia during Cancer Treatment
Journal of Clinical Oncology (ASCO and IDSA), 2018. Antimicrobial Prophylaxis for Adult Patients With Cancer-Related Immunosuppression: ASCO and IDSA Clinical Practice Guideline Update
Annals of Hematology (AGIHO/DGHO), 2022. Management of herpesvirus reactivations in patients with solid tumours and hematologic malignancies: update of the Guidelines of the AGIHO
Journal of Clinical Oncology (ASCO), 2020. Hepatitis B Virus Screening and Management for Patients With Cancer Prior to Therapy: ASCO Provisional Clinical Opinion Update
Lancet Infectious Diseases (ECIL 7), 2019. Guidelines for the management of cytomegalovirus infection in patients with haematological malignancies and after stem cell transplantation
New England Journal of Medicine, 2017. Letermovir Prophylaxis for Cytomegalovirus in Hematopoietic-Cell Transplantation
Lancet Infectious Diseases (ECIL-10), 2026. Community-acquired respiratory virus infections in patients with haematological malignancies or undergoing haematopoietic cell transplantation

Medical disclaimerThis information is for educational purposes only and does not replace medical advice.

Last updated: 3 October 2026← Back to the list

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