Tiny genetic mutations were enough to create a virulent form of chlamydia that causes serious sexual disease in men, researchers say.
An international study found the strain that causes lymphogranuloma vernerum (LGV) is very similar to other forms of chlamydia, past and present.
They also concluded that as few as two gene differences might markedly alter the ability of the disease to thrive.
The decoding of the chlamydia genome is reported in Genome Research.
LGV, which until this century was rarely seen in Europe, causes serious inflammation of the rectum and if untreated can lead to permanent problems.
Symptoms of LGV
Swollen groin lymph nodes
The team looking into the various strains included researchers from the Wellcome Trust Sanger Institute, the University of Southampton Medical School, University College London and the University of California, Berkeley.
They found that this recent form of LGV was virtually identical to one isolated 40 years ago, suggesting that "we are not facing a novel, more dangerous organism".
But they also found that the LGV strain was very similar to another form of chlamydia that causes an eye infection, so-called chlamydia trachomatis.
"Chlamydia trachomatis has almost 900 genes and we found fewer than 10 that differed significantly between the trachoma and the LGV strains," said Dr Nick Thomson of the Wellcome Trust.
The very different effects of the strains were explained by small-scale DNA mutations. These small but key differences could provide new markers for better diagnosis, the study concluded.
It also suggested that perhaps only two or three gene differences might alter the ability of the disease to thrive once it entered the body.
Chlamydia is one of the most common sexually transmitted infections but it is easily treated with antibiotics once diagnosed.
However it is often referred to as the "silent infection" as most people do not have any obvious symptoms until the disease has spread.
LGV is one of the most severe, invasive forms of the disease.
It was virtually unheard in Europe until 2004, when a first outbreak was reported in the Netherlands.
It has since been found across Europe. Hundreds of cases have been reported in the UK, primarily among homosexual men.
Late last year, a new strategy was launched aimed at encouraging more men in England to come forward for chlamydia screening.
"It's a very interesting piece of research," said Dr Gillian Vanhegan, medical spokesperson for Brook Advisory Centres.
"The prospect that you could alter the gene sequence so it could not be replicated is very exciting indeed."