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Infectious Diseases

Infectious Disease information and resources for the MSK community, including clinicians, patients, and the general public.

What Causes Infections?

An infection occurs when a microorganism — such as bacteria, fungi, or a virus — enters a person’s body and causes harm. The effects of infections occur due to the immune system’s attempt to get rid of the invading organism.

How an infection spreads and its effects on the human body depend on the type of pathogen.

The immune system is an effective barrier against infectious agents. However, pathogens may sometimes overwhelm the immune system’s ability to fight them off. At this stage, an infection becomes harmful.

Some pathogens have little effect at all. Others produce toxins or inflammatory substances that trigger negative responses from the body. This variation means that some infections are mild and barely noticeable, while others can be severe and life threatening. Some pathogens are resistant to treatment.

Types of Infections

Bacteria, viruses, fungi, and parasites are different types of pathogens -- the cause of the infection. They vary in several ways, including:

  • size
  • shape
  • function
  • genetic content
  • how they act on the body

For example, viruses are smaller than bacteria. They enter a host and take over cells, whereas bacteria can survive without a host.

Treatment will depend on the cause of the infection. This article will focus on the most common and deadly types of infection: bacterial, viral, fungal, and prion.