Virus and disease are two terms commonly used to describe illnesses caused by harmful microorganisms. Virus diseases can be caused by a variety of organisms, including bacteria, fungi, and viruses. Some of the common synonyms for virus disease include contagion, infection, pandemic, epidemic, and plague. The term contagion is often used to describe an illness that can be easily spread from person to person. Infection is a term that is used to describe any disease that is caused by a microorganism. Pandemic and epidemic are often used interchangeably to describe a sudden outbreak of an infectious disease that spreads rapidly throughout a population. Finally, plague is a term that is used to describe a particularly severe outbreak of an infectious disease.