How worried should we be? Over 50 people in China have been struck down by a mystery virus. Scientists are racing against time to pinpoint the cause before more become dangerously ill.
Mystery illness causes panic in Wuhan, China
Deadly: Coronaviruses (above) are a broad family of viruses, but only seven infect people. Glossary
Smog - A kind of toxic fog caused by high levels of air pollution.
Metropolis - A very large and busy city.
Pandemic - An outbreak of disease which occurs across a wide geographic area, as opposed to an epidemic, which is confined to a particular region.
Coronavirus - A family of viruses which are common throughout the world. Some cause only mild symptoms, such as a cold, but others can be deadly. Examples include Sars, Mers (Middle East Respiratory Syndrome) and Covid-19.
SARS - Severe Acute Respiratory Syndrome, a disease that makes it very difficult to breathe. Between 2002 and 2003, an outbreak of Sars in Southern China caused 774 deaths in 37 countries. It had a 9.6% fatality rate, and spread from civet cats to humans.
Spanish flu - The 1918 influenza pandemic (involving the H1N1 virus) infected an estimated 500 million people worldwide. Despite the name, Spanish flu is not thought to have originated in Spain.
Chinese New Year - Also known as the Lunar New Year or the Spring Festival, it is the most important date in the Chinese calendar. Celebrations last for up to 15 days.
Vaccine - A biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, but in its weakest form.
Ebola - A group of deadly diseases, found mostly in parts of Africa.