Millions of people die preventable deaths each year from diseases like measles, cholera and malaria, often in countries whose populations are affected by factors including poverty, displacement, climate change and lack of health care infrastructure. Almost all of these deaths are caused by infections carried by viruses, bacteria or parasites that can spread through person-to-person contact, animal-to-person contact or in water, food or air.
A disease outbreak occurs when the number of cases of a particular infectious illness exceeds what would normally be expected in a community, geographic area or season. The number of cases in excess of expectation can vary depending on the size and type of population being affected, the infectious agent, and previous and current levels of exposure.
The case definition used to define an outbreak must be adapted for each individual situation and include criteria describing person (e.g., X nursing home or Y high school), place (e.g., state or county), and time (e.g., week associated with onset of symptoms). It must also include clinical features that characterize the cases in question as distinct from similar illnesses that are not part of the outbreak.
Although the number of outbreaks and richness of causal diseases has increased globally since 1980, improvements in prevention, early detection, response and control are making a difference. In fact, even after controlling for variables indicating bias in disease detection and reporting, both overall outbreaks and diversity of causes by pathogen taxonomy, transmission mode and host type (human specific, protozoan or viral) still exhibit significant increases (figure 1; electronic supplementary material). The same approach to disease diversity commonly used in biogeography studies also shows that the proportion of human-specific and zoonotic diseases has continued to increase with time.
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