Drug-resistant tuberculosis (TB) is becoming increasingly common around the world, according to latest research.
Estonia and Cuba have been cited as two countries where the threat is particularly high.
And Dr Mark Tanaka at the University of New South Wales warns that an even more dangerous type of "extensively drug-resistant" TB is now emerging.
The World Health Organization (WHO) estimates that in 2007 there were 9.27 million new cases of TB around the world, with an additional one in three being carries who do not show symptoms.
The spread of resistance TB is partly caused by patients not completing their full course of treatment. They stop taking the pills when they feel better, so some of the bacteria survive and spread.
To discover how fast drug resistance is appearing, and work out the reproductive "fitness" of different strains, Dr Tanaka investigated infections in Cuba, Estonia and Venezuela.
He says: "We found that the overall fitness of drug-resistant strains is comparable to drug-sensitive strains. This was especially so in Cuba and Estonia, where there is a high prevalence of drug-resistance."