Any disease caused by a microorganism, such as bacteria, that is capable of spreading through a population can be termed an infectious disease. Strictly speaking, an infectious disease is capable of spreading through the general environment by means such as airborne droplets and tainted water or contaminated personal items. Other diseases caused by microorganisms that spread by direct person-to-person contact are more accurately termed contagious diseases.
Using this convention, infectious diseases are often capable of spreading faster and affecting more individuals than are contagious diseases, though both types can cause epidemic outbreaks. For that reason and for the purposes of this discussion, we will understand the term infectious disease to also include those diseases that may be strictly defined as contagious diseases. Historically, epidemics typically involve the outbreak of some specific infectious disease.