It is precisely this capacity for TB to lie dormant that is haunting some chest specialists. They believe that the NHS has been dangerously complacent in failing to learn all the lessons of the huge 1992-93 TB outbreak in New York, which has cost hundreds of millions of dollars. They say TB surveillance has been slack, infection control in hospitals poor, and that the whole problem has not been taken seriously enough.
Privy to the latest research, these experts suspect there is a great deal more TB being spread in Britain than is suggested by official figures. They fear that in a few years, TB, which John Bunyan called "captain of all the men of death", could again be a serious killer, particularly since strains resistant to drug treatment (MDR TB) are now in circulation.
This is not the official view. Developing countries and parts of the United States may have seen dramatic increases in TB, but most professionals, proud of Britain's good reputation for public health, believe the NHS has cracked the problem. The statistics seem to bear out their confidence. Although the number of notified TB cases in Britain rose during the Eighties after falling for the previous 40 years, the figure has stayed steady at around 5,600 new cases for each of the past three years. So, although there is concern that homelessness and rising deprivation provides a breeding ground for TB, the statistics suggest no need for concern.
But the early signs of NHS failure are showing up in people with low levels of immunity (in particular, people with Aids). They are developing the age-old symptoms that claimed so many historic figures, from Keats and Shelley to Nicolo Paganini, in the "white plague". The immuno-suppressed are to TB what canaries were to miners, predictors of unseen dangers. Whereas a healthy person infected with TB may take 30 years to develop the actual disease, someone with HIV can fall ill within a fortnight.
Their susceptibility was demonstrated in 1995, during serious outbreaks of tuberculosis among Aids patients in two London hospitals - St Thomas's and Chelsea & Westminster. In each case a patient had been present on the ward suffering from tuberculosis. By coughing, the patient infected others with multi-drug resistant (MDR) TB. Several died.
Paul Mayho, 26, is one of the few who survived the Chelsea & Westminster MDR outbreak. Eighteen months later, he is still on six TB drugs plus an injection three times a week. "I'll be on medication for the rest of my life. I lost my home, my partner, I was locked in a room for three months. At one point I was given 10 weeks to live. When you already have Aids, it is the equivalent of having two potentially fatal illnesses." The side effects of the drugs are nasty. "Streptomycin makes my face go numb. I suffer terrible insomnia. One of the drugs can cause psychosis if there is a build-up."
Old friends who are HIV-positive shun him, even though he is no longer infectious. "They are frightened of the disease. It's very lonely. I've not found anyone who understands how I feel. Completely filthy. I'm full of such anger about the way I was infected." You can almost hear Keats's own angry line: "Youth grows pale, and spectre thin, and dies."
There have been other unpublicised outbreaks in NHS hospitals. Dr Anton Pozniak, senior lecturer at King's College School of Medicine in south London, has documented one such hospital outbreak. It went unnoticed until months later when scientists spotted that a group of people with TB had a strain with the same molecular make-up - they had all been in that hospital around the same time.
These outbreaks could have been worse. In one hospital in Argentina, 162 patients went down with MDR TB over a three-year period. Some had a TB strain resistant to 10 drugs. Most are dead. The NHS has seen nothing on this scale.
During outbreaks, it is difficult to discover how many people have been infected, beyond the Aids patients who actually develop the disease. There is a skin test, which reveals exposure to TB bacteria, but it is of little help. Most Britons give a positive reading because the test reacts to the childhood BCG vaccination - an injection providing limited protection against TB. In short, until people get sick, we don't know how much newly- transmitted (and possibly drug-resistant) TB is dormant in the population.
So you would expect great attention to be paid to the health of the "tuberculosis canaries", not least because of their vulnerability. Yet we have little reliable information on the incidence of TB disease within this crucial population. Notification rates of tuberculosis in people with HIV may be as low as 30 per cent, according to a paper published a year ago in the British Medical Journal by Dr Meirion Evans, a leading consultant in communicable diseases.
The reason for such reticence is the desire among HIV doctors to respect their patients' privacy. Dr Pozniak explains: "If these doctors notify a case of tuberculosis to public health officials, then it means the patient must be followed up at home and contacts traced. They fear that the confidentiality of the patient with HIV will be broken."
However, preliminary data from an important new study involving several London hospitals is filling out the picture of what may be happening. Dr Richard Coker, a consultant physician specialising in TB and HIV at St Mary's Hospital, London, has discovered a worrying increase in the proportion of HIV-positive patients who have developed TB in the past three years. Dr Coker said: "Last year less than 10 per cent of HIV in- patients being looked after in St Mary's were being treated for TB. This year, half of my HIV in-patients have a diagnosis of TB."
In short, TB infection seems definitely to be on the increase. The TB canaries - the HIV population - are the first victims of NHS failures. "Our preliminary findings suggest that TB control in Britain is not as good we thought it was," says Dr Coker. "This could portend badly for the future. As a healthy person, I may have been exposed to MDR TB. If, in 20 years, I get leukaemia or become immuno-suppressed for other reasons, my TB may activate and I will be sick with an infection which is not susceptible to drugs. This will be as a direct consequence of what is happening now."
Dr Coker's fears are supported by the findings of Dr Diane Bennett at the UK's Communicable Disease Surveillance Centre. People with HIV are 100 times more likely than the general population to develop TB. And when they do, it is 1,000 times more likely to be MDR, the new, drug-resistant strain of TB whose presence suggests recent infection.
How did we get into this mess? First, the public health system moved too slowly. Despite the rapid rise of tuberculosis, including MDR, in third world countries, monitoring at British ports has been inadequate. The port authorities are supposed to tell public health officials to follow up all immigrants planning to stay here for more than six months. But active TB cases are slipping through, says Dr Peter Ormerod, chairman of the Joint Tuberculosis Committee of the British Thoracic Society. "It's an immigration system, not a public health system, with only a quarter of all new immigrants referred to port health units." This is worrying because some immigrants are infected with TB. We know this because the incidence of active TB in black Africans in Britain rose by 135 per cent between 1988 and 1993.
Dr Coker recalls the case of a Somalian man. "He told immigration officers that he had been diagnosed in Somalia with TB. After he went through immigration, he was sent off to a holding building for immigrants. Then he went to two hostels where there were other refugees. He finally turned up with us, because he was severely unwell. By then he had been here for a month and was subsequently diagnosed as having pulmonary TB and as HIV-positive. Many people will have been unnecessarily exposed to TB."
Dr Peter Davies, a leading TB physician at Sefton General Hospital, says: "We have also created the conditions for the spread of TB in hospitals. It is wrong that people with HIV are more often than not being nursed on the same infectious disease wards as TB patients. They may be in separate cubicles, but people will still walk out and mix in day rooms."
Another problem is the availability of nursing care in the community. TB treatment, even for drug-sensitive strains, can involve a six-month course of drugs. If the patient does not comply properly, a more virulent, drug-resistant strain of TB might evolve. In the case of such drug-resistant TB, a patient may, like Paul Mayho, become totally friendless and isolated. Unless a nurse is available that patient may not carry on treatment.
Yet, according to Dr Peter Ormerod, there is a serious shortage of TB nurses. "What happens in a town where you have 150 TB cases a year and one TB nurse, who has six weeks holiday a year? The TB doesn't go away when she's not there."
The NHS is belatedly taking action. Guidance will shortly be published on a nationwide strategy. Some hospitals are considering building geographically separate facilities for TB and HIV patients. A pilot project is under way at Heathrow to improve the monitoring of immigrants, with electronic messaging to doctors replacing the old snail-pace system of tipping them off. New systems are being put in place to protect patient confidentiality so that doctors dealing with HIV-positive patients will improve notification of TB cases.
But all this is being done five years too late. We have yet to calculate the personal cost of this delay. We know that many HIV-positive patients may have died earlier than they otherwise would. In a few years, we may find there are more victims, in the general population, who may spend years in treatment that could have been avoided.
It's a story that is familiar in the history of TB. Machiavelli hit on the truth in The Prince when he remarked that "consumption in the commencement is easy to cure and difficult to understand; but when it has neither been discovered in due time nor treated upon proper principle, it becomes easy to understand and difficult to cure."
The author's 'Tuberculosis - America's Health Riot' is published by the Harkness Fellowship of the Commonwealth Fund for New York.