The word pandemic comes from the Greek word "pan", which means all, and "demos", meaning people, and it refers to the global outbreak of a disease.
An epidemic is defined as an outbreak of a disease that occurs over a wide geographic area, and endemic is a disease or condition regularly found among particular people or in a certain area.
The last time an epidemic was declared by WHO was in 2009, when swine flu was estimated to have killed more than 500,000 people worldwide in the first year alone. WHO received a major backlash over whether the outbreak should have been labelled a pandemic.
Since then the organisation has been cautious about how it defines outbreaks of viruses and illnesses.
Ghebreyesus said the word "pandemic" is not one that should be used lightly or carelessly.
“It is a word that, if misused, can cause unreasonable fear or unjustified acceptance that the fight is over, leading to unnecessary suffering and death,” he said.
People often use the terms epidemic and pandemic interchangeably, not realising the two have different and distinct meanings.
Whether it is the current coronavirus pandemic (the resulting disease is scientifically known as Covid-19), Spanish flu or HIV/Aids, we have all heard pandemic, epidemic and endemic used in a variety of ways.