The outbreak of a new and potentially life-threatening illness among Dublin's population of heroin addicts represents a significant challenge for public health specialists, microbiologists and other doctors throughout the city.

A reflection of the seriousness of the situation is the presence here of an expert from the Centre for Disease Control (CDC) in Atlanta.

The CDC is the world centre for the monitoring and control of infectious diseases; its profile has risen in recent years in response to the near flu pandemic in Hong Kong and the sporadic outbreaks of the ebola virus in Africa. Clearly, the CDC would not have a presence in Dublin if the mystery illness fell into an obvious and well-defined medical category.

What are the possible categories into which this undefined illness might fit? Given that the addicts are presenting to casualty departments with a severe inflammatory reaction at injection sites accompanied by particularly nasty soft-tissue abscesses, an infectious cause must be top of the list of suspects.

If an infectious agent was present in the abscess, one would also expect that it would travel throughout the body, causing a serious illness called septicaemia.

Septicaemia can be fatal and would explain the high mortality among the affected addicts. Early indications, however, suggest that septicaemia is not the mechanism by which this outbreak is causing serious illness and death.

Another possible culprit is a toxin. This is where the purity of the heroin causing the illness comes under the microscope. Could the heroin have been contaminated , either deliberately or inadvertently, with a substance toxic to the body?

The fact that the outbreak so far is relatively localised would point the finger at the possible contamination of a batch of heroin. This could have happened outside the State (a theory supported by a parallel outbreak in Glasgow) or at a local level, possibly when the heroin was cut down prior to distribution.

Public health specialists have a well-tried system for dealing with outbreaks such as the present one afflicting Dublin's heroin addicts. Through a combination of laboratory investigations and clinical fact-finding, they build up strands of evidence to link various cases. So far, the more exact laboratory work has not thrown up definite evidence of a particular infection.

The clinical detective work is a particular challenge in this case, because the population affected is, by definition, at the edge of society and might understandably be reluctant to come forward to investigators.

With this in mind, the department of public health medicine of the ERHA has asked casualty doctors and GPs to be particularly vigilant over the holiday weekend.

They have been requested to look out for signs of severe needle reactions or severe abscess formation amongst intravenous drug users. The ERHA has issued a pro forma to doctors which, it is hoped, will help to track down addicts who may have had a mild version of the new illness.

So far, there have been eight deaths out of a total of 15 cases identified. This is a high mortality rate which illustrates the severity of the illness. Whatever the eventual result of the intense medical detective work, the outbreak represents a significant public health challenge with possible global implications.