Tuberculosis flourishes wherever there is poverty, malnutrition, overcrowding and inadequate health care. The statistics make grim reading: globally, 1.72 billion people - or one-third of the world's population - are latently infected with TB. At any one time, between 9 million and 11 million people are suffering from active TB, 95 per cent of them in Asia, Africa, and Latin America.
Each year, TB claims nearly 3 million lives. Half a million children from the developing world die from it annually. The tragic irony is that TB is preventable and, in most cases, curable.
Now, a new, potentially catastrophic chapter in the history of tuberculosis is unfolding. The established epidemic of TB and the new epidemic of HIV have shown a disturbing tendency to coalesce and to co-infect individuals. It is a dangerous liaison both for those who are co-infected and for those communities in the developing world at risk of TB.
Unlike HIV, which is overwhelmingly transmitted through sexual intercourse, TB is highly infectious and casually contagious. TB can be spread by coughing. And while the spread of HIV can be mitigated and controlled through education and safer sex, it is hard to see what measures can halt TB's relentless spread.
The World Health Organisation (WHO) estimates that 4.5 million people, 98 per cent of them in the developing world, are co-infected with HIV and TB. This is almost certainly an underestimate. There are signs, too, of a growing problem in the developed world.
The implications are grave. Dr Paul Nunn, of London's School of Hygiene and Tropical Medicine, says the risk groups for both epidemics overlap in many countries in the developing world. People with HIV are more likely to develop active TB. And HIV-positive people suffering from active TB are three to four times more likely to die than HIV-negative people with active TB.
It is the speed with which the TB epidemic is accelerating in the developing world that most worries the WHO and experts such as Dr Nunn. Since 1987, the rates of people known to have TB in many sub-Saharan African countries have risen sharply. Tanzania, Burundi and Malawi all report huge increases in the number of cases of active TB. In Uganda, confirmed cases doubled between 1984 and 1987, and in Zambia the Ministry of Health said the number of cases had risen from 7,000 in 1986 to 17,000 in 1990.
According to Dr Jan Andersson, a specialist working in Zambia, this increase is mainly due to the rising incidence of HIV. The WHO estimates that around 60 per cent of TB patients in Uganda and Zambia are infected with HIV. In Africa, TB has already become the prime cause of death in adults with HIV.
The devastating potential of this twin epidemic in Africa was demonstrated when Canadian and French researchers used mathematical models to try to estimate its impact on sub-Saharan Africa. In their most optimistic prediction, assuming a low risk of TB infection and low HIV prevalence rates, the researchers calculated that the number of cases of active TB in the 15-49 age group will increase by two-thirds by the turn of the century.
Their most pessimistic forecasts suggested a 12-fold increase in the cases of active TB by the year 2000. Applied to the population of Kampala, this would mean 2 per cent of the city's population of half a million would develop active TB every year.
The study draws some grim conclusions: 'It appears that HIV infection is, as it were, pushing the epidemiological clock back towards the time of the first encounter of human populations with tubercle bacilli.' It warns that there is every reason to expect the increase in the number of tuberculosis cases to continue well into the 21st century.
TB chemoprophylaxis, administering anti-TB drugs to HIV-positive people to prevent the onset of active TB, could save thousands, perhaps tens of thousands, of lives and simultaneously help limit the spread of the epidemic. But the cost is enormous. The WHO has been conducting a study in Uganda on the feasibility of mass chemoprophylaxis but 'the preliminary results suggest a lot of difficulties'.
'Africa needs more and better TB diagnostic services, more and better anti-TB drugs, more hospital beds, and other services and supplies,' says Dr Mario Raviglione of the WHO's TB programme. 'Developed countries need to provide - must provide - more money to target intervention and control programmes in developing countries and to buy drugs and equipment.'
Although TB has not become a real problem in the developed world, there are disturbing portents. In June, the WHO announced that TB has been rising in nine out of 14 European countries. Much of the increase was attributed to immigration, but a number of cases seem to be linked to HIV, especially in southern Europe. TB infection rates among people with HIV are as high as 10 per cent in Spain and Italy.
In the United States, TB has been rising significantly since 1986. The emergence of multi-drug-resistant TB (MDR-TB) in the past 18 months has been greeted with alarm bordering on panic. MDR-TB appears to infect people with HIV and Aids almost exclusively and has so far resulted in a mortality rate of around 80 per cent. MDR-TB has brought calls for draconian measures from the Centers for Disease Control in Atlanta. It wants patients with TB to undergo 'directly observed therapy', which in effect means court-ordered, involuntary detention for the duration of treatment.
Larry Gostin of the American Society of Law and Medicine claims there have already been 'hundreds' of cases of involuntary detention of patients with TB in New York's inner-city hospitals, and says 'patients have been confined to their rooms, sometimes tied to their beds'.
Despite the gravity of the situation worldwide, doctors and epidemiologists are most anxious about the potential spread of HIV/TB co-infection in Asia, where around two-thirds of the world's TB-infected population is concentrated. 'If HIV starts spreading rapidly in India and Thailand, as it seems to be doing, we will have many, many more cases of active TB to deal with,' Dr Raviglione says.
Dr Ishwar Gilada, secretary and founder of the Indian Health Association, believes the seeds of a public health disaster have already been sown: 'In 1988/89 a survey at a hospital in Bombay found that 2 per cent of TB patients were also infected with HIV. This figure has increased to about 10-15 per cent in the past two years. I have very little hope for the year 2000. India is heading for a major health catastrophe.'
Time is running out all over the world. The WHO has called the epidemics 'an immediate and grave public health and socio-economic threat, particularly in the developing world'.
Neil McKenna is editor of 'WorldAIDS' magazine, published by the Panos Institute.