A disease outbreak occurs when cases of a particular disease occur in excess of expected levels (see Chapter 14, Outbreak investigation). This is usually caused by a pathogen and can be caused either through human-to-human or animal-to-human transmission. It can also be caused by chemicals or radioactive materials.
Infectious diseases often cause disease outbreaks, particularly those that can spread easily from person-to-person or through water, food and air. These include infections with bacteria, fungi, parasites and viruses.
Outbreaks are more likely in countries with poor surveillance capabilities, weak infrastructure and limited communication networks (see Figure 1b, electronic supplementary material). They can also be caused by environmental factors such as climate change, which can alter the ecology of vector species and reduce food security, leading to increased human vulnerability.
Other factors that can contribute to outbreaks include poor water, sanitation and hygiene facilities and limited vaccination coverage. This exacerbates inequalities and increases the risk of infectious diseases impacting vulnerable communities.
Successful outbreak investigations deliver valuable information about microbial behaviour. They can reveal novel modes of transmission, e.g. through contaminated foods, imported animals and swimming pools. They can also inform general control and prevention measures. This can be done by taking contaminated products out of the market, closing factories, limiting contact between affected and unaffected individuals or changing practices that might propagate the infection e.g. changing traditional burial practices or educating people to wash their hands. This is why anthropologists and communication experts are routinely included in outbreak response teams.