The global public health community has seen a number of infectious disease epidemics and pandemics in the 21st century, and new lethal threats are likely to emerge as well. In the past, major epidemics like cholera and plague have returned to modern life, and new threats such as severe acute respiratory syndrome (SARS), Middle East Respiratory Syndrome (MERS), and pandemic influenza have emerged.
A disease outbreak is called an epidemic when the case numbers increase rapidly over a short period of time, usually resulting from human-to-human transmission through direct contact, such as in some sexually transmitted diseases; indirect contact, like sharing a vehicle or item that transmits a disease, such as needles, also known as object-borne transmission; or the spread of a disease by a vector, like mosquitoes, such as in the 2014 Ebola virus outbreak in West Africa. The World Health Organization (WHO) declares a disease pandemic when its growth rate is exponential and crosses national or international boundaries, as opposed to a localized epidemic or endemic disease that is consistently present in a region.
An epidemic can also be triggered by factors that create conditions favorable for disease transmission, such as droughts and floods that can displace populations and allow contact with an infective pathogen; war, civil unrest, or natural disasters that can cause mass evacuations and lead to social disruption and displacement, making it easier for people to come into contact with a disease; or the microevolution of a clone or strain of a bacteria or virus that causes a disease, which allows it to spread easily from person to person. These events have caused significant human and economic loss, with both short- and long-term effects on development.