In 2002-2004, a very bad cold called SARS (which stands for Severe Acute Respiratory Syndrome) made people very sick in parts of Asia, North America, and Europe.
SARS is caused by a virus that attacks people's lungs and makes it hard for them to breathe. The virus spreads from person to person when someone who's sick coughs or sneezes and the droplets from their mouth or nose go into someone else's mouth or nose.
To try and stop the sickness from spreading, many countries put in place special rules called "quarantines." This means that people who have been in contact with someone who is sick are asked to stay home and not go to school, work or public places for a while to make sure they don't get sick too.
Doctors and scientists worked really hard to understand SARS so that they could create a special medicine called a vaccine to help prevent people from getting sick.
Eventually they were able to contain the spread of SARS and stop the outbreak from becoming worse. But it was a scary time for many people and we all learned how important it is to take care of our health and follow special rules to prevent the spread of sickness.