A third mutant strain of Covid-19 was detected in Brazil this week.
The Brazil variant was discovered by Japanese scientists from samples taken from four travellers from the South American country.
Coming in the month after variants from the UK and South Africa came to the world's attention, there are concerns the South American strain could also be more infectious than previous versions of the novel coronavirus.
While the latest strain is new and scientists haven't had time to study it in any great detail yet, its discovery is the latest alarming update regarding a disease which has now killed almost two million people worldwide.
Below is a guide to the mutant coronavirus strains, explaining everything you need to know and how worried we should be.
What is a mutant strain?
Since Covid-19 - which is technically called Sars-CoV-2 - was found around a year ago it has mutated thousands of times.
While that sounds alarming, it is to be expected for a virus that has proved to be so wildly 'successful'.
Most of those mutations are "passengers" and have little impact on the way the virus works.
Once in a while the coronavirus will mutate in a way that helps it survive and reproduce however.
If the settings are right - as in the cases of the UK, South Africa and Brazil strains - then the mutated virus can start spreading like wild fire.
There are two notable ways in which the novel coronavirus can mutate.
Both happen in the spike protein, which is like a key that the virus uses to enter cells inside a body.
The first is called mutation N501 and alters part of the spike protein called the "receptor-binding domain", which is where the virus first makes contact with the body's cells.
The mutation helps the virus enter the cell more easily.
The other mutation is called H69/V70 deletion, which has been found several times including in mink.
Work is ongoing to find out how this mutation will change the way the virus interacts with human bodies.
Which countries have found mutant strains?
The UK
The UK's own mutant strain has ripped through the country since it was first detected in September.
Referred to as H69/V70, the virus may be 70 per cent more transmissible than previous versions, the government has said.
What is unusual about the UK's strain - which has a N501 type mutation - is how highly mutated it is.
It is possible that it emerged in someone who had a weakened immune system who could not beat back the virus.
South Africa
On December 18 South African authorities announced they had detected a mutated strain which was spreading rapidly through three provinces.
Named 501Y.V2 - because of the N501 type mutation - the variant is thought to be more infectious than previous versions.
While both the UK and South Africa strain share the same variant, they are different.
The South Africa version carries two other mutations in the spike protein not present in the UK strain.
One is named VOC-202012/01, with VOC standing for "Variant of Concern."
Brazil
A new strain of coronavirus believed to have originated in Brazil has been identified in the past few days.
The latest mutation was discovered in Japan after four people travelled from the Amazonas state to Tokyo earlier this month.
The Japanese National Institute of Infectious Diseases (NIID) says at the moment there is "no proof" that the variant is more infectious than previous Covid versions.
However, Cambridge University microbiology professor Ravi Gupta said the Brazilian variant has three key mutations that "largely mirror" some of those in the hyper-infectious South African variant "hence the concern".
The variant has the same N501 mutation as the other two strains.
Japan announced the detection of the new variant on Sunday and reported it to the World Health Organisation.
The detection is more bad news for a country which has been battered by the virus.
Officially more than 200,000 Covid-19 deaths have been recorded in Brazil - the second highest number behind the US.