Cells throughout their life cycle start to lose connections with other cells, and then our immune system works out to kill those cells. This is known as “apoptosis” from a Greek word meaning “falling off,” as leaves from a tree.

During the plague epidemic in the 14th century, ships arriving in Venice from infected ports were instructed to sit at anchor for 40 days before landing. This practice, called quarantine, was derived from the Italian words Quaranta Giorni which mean 40 days. Quarantine is the separation of a person or a group of people who although do not have symptoms of illness may have been exposed to a transmissible disease, from other people to prevent the spread of disease. The concept of quarantine is introduced by Avicenna (the European name for Abu ‘Ali al-Husayn ibn Sina) a significant Persian polymath who helped to elucidate the contagious nature of infectious diseases.