Infectious diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites or fungi; the diseases can be spread, directly or indirectly, from one person to another. 

Many organisms live in and on our bodies. They are normally harmless or even helpful but under certain conditions, 

These organisms may cause disease
Many Infectious Diseases, such as measles and chickenpox, can be prevented by vaccines. 
Frequent and thorough hand-washing also helps protect you from most Infectious Diseases. Some infectious diseases can be passed from person to person
Some are transmitted by bites from insects or animals. And others are acquired by ingesting contaminated food or water or being exposed to organisms in the environment.
The infectious disease department forms an integral part of a hospital. 
The department strives to create awareness amongst doctors and staff regarding optimal use of antimicrobial agents through lectures, workstations, case presentations.
 The infectious disease service provides round-the-clock telephonic assistance for decision making regarding post-exposure prophylaxis (treatment) for outpatients attending the emergency services for dog/cat/animal bites. 
Assistance is also provided for decision making regarding needle-stick injury management for health care workers exposed to a blood-borne infectious disease.
The infectious disease service evolved as a subspecialty of internal medicine and focuses on quality management of infectious diseases, 
particularly tropical infections, tuberculosis, human immunodeficiency virus (HIV) infection, organ transplant-related infections 
Invasive fungal infections and fevers of unknown origin. In this era, patients with comorbid illnesses can have complicated infections which require multidisciplinary management

. Infectious disease physician often is a part of f team of doctors who treat such critically sick patients.