The sickness of a housing estate is not so different. There, too, the warning symptoms are minute but clear. Sometimes they don't develop further. But sometimes they lead to the collapse of a whole community, the flight of families, police patrolling in Land Rovers with grilles over the windows. As the Germans say, Wehret den Anfangen: beware of the first signs.
For me, in the days when I used to go electioneering in the West of Scotland, the symptom was a door. The close (entry passage) might be clean, the common stair swept, most of the apartment doors in good order. But sometimes there would be one door with a bruised look - painted-over cracks in the panels, an unevenness where a lock had been replaced.
Here dwelt somebody who got into his home with a boot rather than a key.
If the repairs were fresh, the disease was usually about to spread. Returning a few months later, one might find a gruesome change in the patient's appearance: rubbish and graffiti, the smell of urine, and - worst of all - more bruised doors as "respectable" tenants fled the stair and were replaced by families thrust in by the council as emergency cases.
If the area was vulnerable to infection because of unemployment and general abandonment, as many of the big peri-urban housing schemes were, the plague would multiply until block after block had its ground-floor windows nailed over with hardboard and the kerbs and grass verges were scattered with dismembered cars.
It's easy to take refuge behind a figure of speech, and describe this disaster by a medical metaphor. But the guy with the boot is not a virus. He is a human being who ruins the lives of other human beings. He and often his family - the women and children he leaves behind him when he finally kicks his way out of the universe - are the phenomenon we call "anti-social tenants" or "neighbours from hell".
A great row is going on in Glasgow about Councillor Jim McCarron, the city's housing convener. He lost whatever cool he may have possessed when the decision was taken to bulldoze 200 houses in South Balornock, a district which was renovated only 10 years ago at a cost of pounds 2.7m.
The housing scheme was provided with play-parks and flower-beds and nice paved patios, and even won an urban regeneration award. Now it has been reduced to uninhabitable desolation by a small minority of fire-raising, vandalising tenants.
Mr McCarron declared that enough was enough. He proposed that from now on "problem tenants" should be segregated and forcibly resettled in special ghetto estates, surrounded by wire fencing and patrolled by security guards.
A roar of protest followed. Social policy experts objected that "punishment blocks" had been tried in the city 30 years ago and been an utter failure. Others asked who the heck would wish to live in the streets around such a ghetto.
Then came the counter-protest. Mr McCarron denounced his critics as middle-class do-gooders who had no idea what it was like to live with such neighbours: "It's time we acted on behalf of all the decent council tenants of Glasgow. What we are going to do with people who deal drugs and vandalise other people's property is marginalise them."
One of the leaders of a Glasgow tenants' association called the McCarron plan "lunacy". That seems a bit hard. A posturing, populist counsel of despair would be nearer the mark. For many years, local authorities all over Britain have been dumping problem families into ghetto housing. In the remote past, such solutions did occasionally produce a sort of social armistice. North Islington in the 1930s harboured the fearsome "Campbell's Bunk", a no-go area which resembled a Hogarthian thieves' kitchen of the 18th century. It developed its own informal structures and self-discipline, and on the whole kept its wildness to itself.
At the 1937 coronation, the Bunk was admired for its street banquets under "Poor But Loyal" banners. But solidarity like that belongs to a social age as remote as the Pleistocene.
Countless gloomy precedents show what a "McCarron's Bunk" would be like today. It would never develop into a self-regulating community but would remain at war with itself and everyone else. Its drug-based economy would spread violence and organised crime across the city.
Containment by segregation is no longer an answer, if it ever was. Wire and guards? Why not go the whole hog and build a labour camp in the Lanarkshire hills, or shoot the lot?
Having said all that, there is something in Mr McCarron's outburst that can't be buried under high-minded ridicule. Talking about "nightmare neighbours" is an uneasy, inhibited process, and this is because an ideology and a set of facts seem to be in conflict. The ideology is that crime - at the level of vandalism, underage robbery and housebreaking, street violence or smack-peddling up the close - is primarily caused by poverty, family breakdown, joblessness and "exclusion". The facts are that most of those offences emanate from an extraordinarily small number of households.
Mr McCarron said wrathfully that "a minuscule percentage of tenants" causes the problems, and he is right. Anybody who spends time in a local criminal court knows how much time is spent dealing with members of the same few families from the same few streets. It is true that many, probably most people who live in sink housing estates transgress and fiddle a bit to get by and feel human. But the behaviour that makes a whole block or street uninhabitable comes overwhelmingly from the very few - and the very identifiable.
Does this explode the view that poverty and lack of opportunity are the sources of crime? No, it does not. What it says is that the connection between deprivation and crime is indirect. As social workers know, it's intimate, human factors which dictate that one individual or family loses control in hard times, while the neighbours do not.
Dachau-on-Clyde, internment for the anti-social, is not an answer but a nightmare. Better by far is the Dundee experiment reported last week in the Scotsman, run jointly by National Children's Homes and the council. There a tenement block in one corner of a housing estate has been turned into a unit for problem tenants. Up to seven families at a time are given night-and-day "intensive care" by a permanent staff of 15 social workers and counsellors.
The scheme has been running for a year, and so far it works. Several families have been returned to normal housing, and have apparently learnt to cope.
Almost as striking is the attitude of other families living next to the special block. They expected the worst but now find its children better behaved, if anything, than those in the other tenements.
No wire, no guards. The Dundee model, with all its staff, costs a lot of money and can help only a few families at a time. But surely money was never better spent, and this government, if it means what it says about fighting "exclusion", should extend this sort of experiment to every troubled housing estate in the land.