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Plague is an infectious disease caused by bacteria usually found in small mammals and their fleas.

It has an extremely high fatality rate and is very infectious, although it can be treated by antibiotics if it's caught early.

There are three forms of plague infection: pneumoic plague, septicaemic plague and bubonic plague, the most common form.

Bubonic plague was known as the Black Death in medieval Europe, where an outbreak brought entire civilisations to their knees and decimated the world's population.

Black Death is spread through the bite of infected fleas, whereas pneumonic plague, the most contagious form, develops after a bubonic infection.

Pneumonic infections can then be spread through the air, while septicaemic plague occurs when infection spreads through the bloodstream. The Black Death, a widespread bubonic plague infection, peaked in Europe between 1346 and 1353.

It was one of the most devastating pandemics in human history, leaving an estimated 75 - 200 million people dead in Eurasia.

This fatality rate represents a staggering 30-60 per cent of the European population at the time.

After the plague, society experienced a series of marked changes, including a rise in religious fanaticism.

Lacking the medical knowledge to understand the pandemic, some groups blamed Jews and lepers for the outbreak - resulting in mass killings throughout Europe.

It took around 300 years for global populations to return to pre-plague levels after the outbreak. The three different types of plague all refer to different ways the disease can be spread.

In bubonic infections, plague-causing bacteria can be transmitted between animals and fleas, with infected fleas then passing the disease on to people through bites.

Infected people may then develop pneumonic plague once their bubonic infection becomes advanced.

Lung-based pneumonic plague can then sometimes be transmitted through the air between sufferers.

Following a pneumonic or bubonic infection, people can then develop septicaemic plague, which occurs when the infection spreads through the bloodstream. The Great Plague of London struck the city in 1665 and lasted well into 1666 and was the last major outbreak to hit England.

Across Europe, the plague was endemic in major cities, periodically erupting in pockets across the continent for centuries.

But the outbreak of 1665 was of a proportion far greater than had been seen for some time.

It is not known where the plague that hit London originated from but it is thought most likely to have come across on a Dutch ship.

While deaths began in late 1664 they struck in earnest at the start of 1665 and were rampant by the summer, causing the King to flee the city in July.

At its height the disease was claiming 7,000 lives a week, according to conservative measurements.

The outbreak did not fully come to an end until September 1666, coincidentally as the city was struck by another tragedy, The Great Fire.

Some believed that this is what caused the end to the plague but others argue that it had already virtually died out by the time it struck.