The Justinian plague and the Black Death are two of the deadliest plagues in human history – both responsible for killing almost half of the European population. Now, an international research team has discovered that both plagues were caused by different strains of the same bacterium.

This is according to a study recently published online in The Lancet Infectious Disease.

The Justinian plague occurred in the sixth century. It is estimated to have have killed between 30 and 50 million people as it spread across Asia, North Africa, Arabia and Europe.

The Black Death, also known as the Bubonic plague, occurred almost 800 years later. Between 1347 and 1351, this plague was responsible for the death of approximately 50 million Europeans.

The researchers say very little is known about what caused the Justinian plague and whether it was linked to the cause of the Black Death.

With a view to finding out, the investigators analyzed short DNA fragments from the teeth of two 1500-year-old victims of the Justinian plague who were buried in Bavaria, Germany.

The researchers say that to date, these DNA are the oldest pathogen genomes found.

The researchers reconstructed the genome of the oldest Yersinia pestis – the bacterium responsible for the Justinian plague – using the short DNA fragments.

The plague genome was them compared with over 100 contemporary strains of the bacterium.