← Back
Immunity and infection prevention

What is an infection

The main article (“What is an infection?”) is about germs. This topic looks at infection itself: how a germ gets into the body, how infection differs from colonisation, and why it looks different in leukaemia.

Last updated: ~1 min read
A scale with germs and a shield.
Figure 1.Infection is a loss of balance.

01Short answer

An infection is when a germ (a bacterium, virus, fungus or parasite) enters the body, multiplies and causes harm. Many germs live on and in us; this is colonisation, and it is usually harmless. Infection starts when a germ crosses a protective barrier: the skin, a mucous membrane or a catheter. In people with leukaemia, many infections start from our own germs, the flora of the mouth, gut and skin. When immunity is weak, the body’s response (redness, swelling, pus) can be faint, so fever is often the only sign.

02Simple explanation

Infection: a germ gets in and does harm.

Colonisation: a germ is present but does no harm.

Often from our own germs.

03Scientific explanation

Internal source: during neutropenia, some bloodstream infections come from gut and skin germs.

Weak inflammation: with few neutrophils the signs of inflammation are faint, so infection can stay “hidden”.

04What happens in the body?

A balance

Infection is a loss of balance between a germ and the body’s defence.

Colonisation and infection

The same germ can live harmlessly on a surface but cause infection once it enters tissue.

Bacteria on a surface and in tissue.
Figure 2.Colonisation differs from infection.

Inside and outside

Germs come both from our own flora and from the surroundings.

A human silhouette with arrows.
Figure 3.Germs come from inside and outside.

05How an infection develops

Four steps.

Arrival — A germ arrives.1ArrivalA germ arrives.
Entry — It crosses a barrier.2EntryIt crosses a barrier.
Multiplying — It grows in tissue.3MultiplyingIt grows in tissue.
Response — The body reacts.4ResponseThe body reacts.

06Types of germ

TypeExample sites
BacteriaBlood, lungs, urinary tract, catheter
VirusesAirways, herpes, CMV
FungiMouth, lungs, blood
ParasitesRare; with travel or food

07Key terms

Infection
A germ entering and causing harm.
Colonisation
A germ present without harm.
Pathogen
A germ that causes disease.
Flora
The body’s normal germs.
Inflammation
The body’s response to injury and infection.

08Key points

  • Infection is harm done by a germ.
  • Colonisation is usually harmless.
  • Many infections come from our own germs.
  • With weak immunity, signs are faint.
  • Fever is often the only sign.
  • If you feel unwell, tell your team.

09Frequently asked questions

Can my own germs make me ill?

Yes, when defences are weak; that is why hygiene matters (“Hand and body hygiene”).

Does infection always cause fever?

No; steroids and older age can hide it.

Is colonisation treated?

Usually not, but in some cases your doctor may take special steps.

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.

  1. Can my own germs make me ill?

  2. Does infection always cause fever?

  3. Is colonisation treated?

My questions

12Sources

  1. MedlinePlus (U.S. National Library of Medicine). Bacterial Infections
  2. MedlinePlus (U.S. National Library of Medicine). Viral Infections
  3. National Cancer Institute (NCI). Infection and Neutropenia during Cancer Treatment
  4. American Cancer Society. Stem Cell Transplant Side Effects
  5. American Cancer Society. The Stem Cell Transplant Process
  6. Biology of Blood and Marrow Transplantation, 2009. Guidelines for preventing infectious complications among hematopoietic cell transplantation recipients: a global perspective
  7. InformedHealth.org (IQWiG), NCBI Bookshelf. In brief: How does the immune system work?
  8. InformedHealth.org (IQWiG), NCBI Bookshelf. In brief: The innate and adaptive immune systems
  9. MedlinePlus (U.S. National Library of Medicine). Immune System and Disorders
  10. Cancer Research UK. The immune system and cancer
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
Last updated: ← Back to the list

Open the interactive site →