A contagious disease is one that can be spread from person to person. It can be shared through direct contact, like touching someone who is infected; indirect contact, like touching an object that is contaminated; or droplet contact, like inhaling the droplets expelled by an infected person talking, coughing, or sneezing. A contagion, like a virus or bacteria, is the agent responsible for causing contagious diseases. The disease itself can also be called a contagion.

Contagious diseases can be spread by an infected person for varying lengths of time. With a viral infection, the length that one is contagious can vary depending on the virus. The flu, for example, also known as influenza, is usually infectious from the day before symptoms develop until five to seven days afterward. The common cold, on the other hand, can be infectious from a few days before symptoms appear until all of the symptoms have disappeared. This usually lasts about two weeks. An infected person is usually most contagious during the first two to three days of a cold, when symptoms are the worst.

In some cases, contagious diseases can become an epidemic or even a pandemic. An epidemic is a sudden disease outbreak that affects a large number of people in a particular region, community, or population. In an epidemic, the number of people affected by the disease is larger than what is normally expected. A pandemic is a disease epidemic that has spread to a large group of people across a large region, multiple nations, or continents.

For example, in 2014, the Ebola virus caused an epidemic that killed thousands of people, largely in western Africa. It was a propagated epidemic as the virus spread through human contact with contaminated body fluids. As families tended to sick relatives and other caregivers helped ill patients, many were exposed to the virus and became sick themselves.

An influenza epidemic turned into a deadly pandemic in 1918, when it spread around the globe. In World War I, the fact that troops were moving and spent much of their time in close quarters helped fuel its expansion. Of the five hundred million people who were infected, at least fifty million died.

While a disease might be contagious, that does not mean it will infect every person who comes into contact with it; it may even have different effects on different people. Whether someone develops symptoms is controlled by their level of immunity, which can be active or passive.

Active immunity means one’s body produces antibodies to fight off the virus or another disease-causing agent. This happens after a specific disease is introduced to the body, whether through previous infection or a vaccine. This immunity usually is permanent and will last a person’s entire life.

Passive immunity means the antibodies were transferred from one person to another, like from mother to baby through the placenta or breastfeeding. Passive immunity usually lasts a few weeks to a few months.