Factors predisposing a patient to infection can broadly be divided into three areas:

• People in hospitals are usually already in a ‘poor state of health’, impairing their defense against bacteria – advanced age or premature birth along withimmunodeficiency (due to drugs, illness, or irradiation) present a general risk, while other diseases can present specific risks – for instance, chronic obstructive pulmonary disease can increase chances of respiratory tract infection.
• Invasive devices, for instance intubation tubes, catheters, surgical drains, and tracheostomy tubes, all bypass the body’s natural lines of defence againstpathogens and provide an easy route for infection. Patients already colonised on admission are instantly put at greater risk when they undergo invasive procedures.
• Patients’ treatments can leave them vulnerable to infection – immunosuppression and antacid treatment undermine the body’s defences, while antimicrobial therapy (removing competitive flora and only leaving resistant organisms) and recurrent blood transfusions have also been identified as risk factors.