Healthcare-associated infections (HAIs) occur in a healthcare facility while a person receives care for another ailment. This category also includes work-related infections among healthcare staff.

HAIs significantly increase the cost of health care. Extended stays in healthcare institutions, increased disability, increased costs of medications, including specialized antibiotics, and prolonged recovery times add to the patient's expenses and the healthcare institution and funding bodies. Common HAIs include:

• Central line-associated bloodstream infections (CLABSI)
• Catheter-associated urinary tract infections (CAUTI)
• Ventilator-associated pneumonia (VAP)
• Methicillin-resistant Staphylococcus aureus infection (MRSA)
• Surgical site infections (SSI)
• Clostridium difficile infection.

For all these types of infections, symptoms may appear within forty-eight hours of admission or thirty days after discharge.

HAIs may be exogenous or endogenous in origin. Exogenous infections occur when a pathogen enters a patient's body from their environment. For example, a healthcare worker can spread the infection due to poor adherence to infection control practices. In contrast, the body's normal flora may cause an endogenous infection; they may act as opportunistic pathogens when the host is susceptible.

The body's normal flora is a stable population of microorganisms living on the surface and deep layers of skin, saliva, oral mucosa, gastrointestinal, and genitourinary tracts. Under normal conditions, they help maintain health, but an infection may result when the body is stressed or immunity is compromised.