Outbreaks of infectious diseases are rising around the world, although deaths are dropping. Here we show several snapshots that capture these trends. Recently in the U.S., the number of people getting sick from certain ailments has risen. Worldwide, the number of people killed from many types of infections has decreased over the long term, although the trend varies at different economic levels. Overall, disease outbreaks, a measure that includes both sickness and death, have become more frequent, with more varied causes.


Infectious Disease: Illness caused by microorganisms, such as bacteria, viruses, parasites or fungi, that can spread from one person to another or from an animal to a person.

Endemic: Describes the baseline level of a disease usually present in a community.

Epidemic or Outbreak: An increase, often sudden, in the number of cases of a disease above normal levels in a region. An outbreak sometimes refers to an increase in a smaller geographical area.

Pandemic: An epidemic that has spread across several countries or continents and usually affects a large number of people.

Zoonosis: A type of infectious disease that originates in vertebrate animals and moves to people. It can be spread by direct contact or carried from animals to humans by a vector such as a biting insect.

Measures of Disease

Mortality: The number of deaths caused by a disease in a population at a particular time.

Incidence: The number of new cases of a disease in a population at a particular time.

Credit: Jen Christiansen; Sources: Amanda Hobbs (research); “Summary of Notifiable Infectious Diseases and Conditions—United States” reports for 2014 and 2015, in Morbidity and Mortality Weekly Report; 2016 Annual Tables of Infectious Disease Data, National Notifiable Diseases Surveillance System, Centers for Disease Control and Prevention, 2017 (Legionnaires’, mumps, pertussis and hepatitis C data); Sexually Transmitted Disease Surveillance 2016. CDC, September 2017 (STD data); Global Health Estimates 2015: Deaths by Cause, Age, Sex, by Country and by Region, 2000–2015. World Health Organization, 2016 (mortality data); “Global rise in human infectious disease outbreaks,” by Katherine F. Smith et al., in Journal of the Royal Society Interface, Vol. 11, No. 101; December 6, 2014 (outbreak data)