A disease outbreak occurs when a number of people in a region get sick from the same disease or infection. This is different from a normal pattern of illness in the community (also known as the baseline level of illness). It is identified by a cluster of cases, with a clear association between them shown by an investigation. The cluster can be a small group within a town or city, or it may be more widespread and involve people across the country or even internationally.
The frequency of global infectious disease outbreaks has increased over the past 3 decades, with predictions that this trend will continue owing to factors such as increasing global mobility, rapid urbanisation, and climate change. The types of outbreaks have also changed. Highly pathogenic or highly transmissible diseases, such as Ebola (DRC and DR Congo), Marburg virus haemorrhagic fever, and monkeypox (Nigeria) are more often reported, whilst vector-borne diseases such as dengue, chikungunya and Zika are increasingly common, particularly in tropical regions.
Pathogens spill over into human populations from animals or their food, and through air, water, and other media. They are also more likely to emerge in situations of global vulnerability such as conflict, natural disasters, and humanitarian emergencies – where lack of access to clean water and other basic services facilitates disease transmission and increases the impact of an outbreak. The Smithsonian’s National Museum of Natural History explores the science behind these global risks in its exhibition ‘Outbreak: Epidemics in a Connected World’.