What are infectious diseases?

Infectious diseases are human illnesses caused by microbial agents that can be transmitted from one person to another, either through direct contact or through the environment (e.g. infected air, water, food, objects, and surfaces). Some microbes are helpful and are required for normal bodily function, particularly in the gut. Others, under the right conditions, cause human illness. These illnesses range from benign conditions like folliculitis or skin infections, to long-standing and chronic infections (e.g., tuberculosis, leprosy, Lyme disease), to rapidly fatal illness (e.g., SARS, septic shock).