There have now been 43 cases in Scotland, most in Glasgow with 20 deaths, more than 16 suspected cases in England and Wales, with at least nine deaths, and 15 cases with eight deaths in Dublin.
It is thought that a bacterium called Clostridium novyi, which thrives in dead flesh, is causing the problem.
A spokesman for the Greater Glasgow Health Board said: "Our suspicion is that these clostridium have been in the heroin that the patients have been injecting.
"We might be talking about bacteria, but basically this is a human tragedy."
He said that the new cases which have emerged in recent days meant that the "bad batch" of heroin had obviously not yet been used up. The Public Health Laboratory Service (PHLS) has been working with the Centers for Disease Control in the US to track down the cause of the illness.
There have now been 43 cases in Scotland, most in Glasgow with 20 deaths, more than 16 suspected cases in England and Wales, with at least nine deaths, and 15 cases with eight deaths in Dublin.
Illness in humans caused by this particular bug is rare, although vets have encountered cases in animals.
Its spores can lie dormant in soil for months or years, and only become active in very particular circumstances, as the bacteria can only live in an oxygen-free environment.
The illness has only affected drug users who inject heroin directly into muscle as opposed to a blood vein.
Experts believe that the injection site may have dead tissue close by, which has no oxygen supply and allows the spores to activate and grow.
They also speculate that the citric acid often added to the drug before injecting may help "activate" the spores.
The bacteria themselves stay close to the injection site - but powerful toxins they release travel around the body, leading to death in many cases.
Professor Brian Duerden, of the PHLS, said: "Clostridium novyi is a very virulent organism - when it causes infections it causes very severe infections."
Doctors are urging anyone with symptoms to get to hospital quickly - but warn that the infection is hard to treat, even with modern antibiotics.
But surgeons may be able to cut away the dead tissue containing the bacteria.
Early symptoms are swelling or inflammation around the injection site, followed by abscesses that get progressively worse over several days.
Doctors are urging anyone with symptoms to get to hospital quickly - but warn that the infection is hard to treat, even with modern antibiotics.
But surgeons may be able to cut away the dead tissue containing the bacteria.
The illness in Glasgow emerged at the beginning of May, with the cases in northern England emerging soon afterwards.
The Dublin cases have followed a similar pattern on illness, but most of the victims have been men.
Tests at the government's research centre in Porton Down ruled out anthrax, which scientists suggested may have caused the Glasgow outbreak.
The clostridium family of bacteria include other strains which cause the disease tetanus and some cases of food poisoning.