Scientists have discovered that factors such as human immunity and drug resistance are less important to the success of bacterial spread than previously thought.

, most of the variation in the spread of bacterial pathogens occurs simply by chance.

The team from Imperial College London studied three famously deadly species: 

, which kills 1.8 million people around the world every year, and 

, which in its drug-resistant form is better known as MRSA. They compared the genetic make-up of these bacteria with a computer simulation which allowed them to test various evolutionary scenarios.

They found evidence that bacterial communities mirror the social life of the humans they infect. For example, close family members or friends in the same class at school or nursery are more likely to share an infection due to increased contact. After accounting for these differences in the opportunities for person-to-person spread, the researchers were amazed to find that there was little evidence for differences in the ability of pathogen strains to spread.

Dr. Christophe Fraser, from Imperial College London, a Royal Society University research fellow and one of the authors, says: "Microbiologists have assumed for some time that some disease strains spread more successfully than others. In fact we found that the variation in the communities we studied could be explained by chance. This was surprising, especially considering all the potential advantages one pathogen can have over another, such as antibiotic resistance and differences in host immunity."

Dr. Bill Hanage, from Imperial College London, and also one of the authors, says: "When we look at a sample and see that some strains are much more common than others, it's tempting to think that there must be something special about them. In fact, they could just be the lucky ones, and that's what it looks like here. Most of the variation in the spread of these pathogens can be explained by chance alone."