For a brief moment the other day David Cameron looked less like a benign politician in a politically correct age and more like the sort of puritanical fanatic of yesteryear who enjoyed putting unmarried mothers and orphans caught stealing apples into the stocks, or worse.

Setting out his plans to demonise those who claim sickness benefit for what he glibly calls "treatable conditions", Mr Cameron destroyed any illusion that the Conservative Party is concerned about the welfare of the people.

His proposed strategy for reclaiming some of the weekly £10 million of benefits paid to those too unwell to work for reasons of obesity, or drug and alcohol dependence, has more in common with the Victorian workhouse ethic than anything appropriate for our more enlightened times. In fact, I should perhaps not have used the word workhouse, lest it catch the eye of a Tory airhead who might think it an idea worthy of resuscitation.

Apparently there are around 100,000 people in receipt of benefits for these kinds of incapacity. Most of us would consider that a reason for soul-searching rather than penny-pinching. Why is the number so high? Are the sufferers getting the help they need?And if not, does the root of the problem lie with those in the medical profession, or with claimants' reluctance or inability to ask for or follow advice?

Whatever the truth, one thing is certain: this will not be a good week to be seriously overweight and unemployed, let alone alcoholic or drug addicted. It's bad enough to endure these afflictions without also being castigated as work-shy.

Alastair Campbell is only one of this scheme's critics to point out that alcoholism is an illness, not a lifestyle choice. As he said: "People who are walking around London and the rest of the country today, as they walk over people sleeping on the streets, just ask themselves if these people really, really chose to be there."

The same is true of drug addicts, whose self-loathing is often second only to the need for another fix. Meanwhile the obese are probably the least understood and most despised of all. There is no glamour about someone who cannot resist food, or has trouble climbing stairs and getting into a bath.

Heavy drinking and drug-use are often portrayed as alluringly dangerous and rebellious, but the same is never thought of a bucket of chicken nuggets. And while there are shelves of memoirs about giving up drink and drugs, the obese have very few laureates.

Theirs, indeed, is arguably the most pitiful condition, being visible to everyone as soon as they step out of their front door.

While one can understand an employer being wary of taking on someone with a history of substance or alcohol abuse, for fear they relapse, is it not possible that the grossly overweight find it even harder to get a job, not because they are insufficiently keen or qualified, but because of prejudice?

Given the choice between two equally good candidates, one suspects the thinner will usually win the post. Yet it's safe to say that inside almost every flabby person there is a thin one begging to get out. Obesity, and a person's inability to tackle it, is as much a clinical crisis as that of the alcoholic or junkie.

Conditions like these demand understanding and a redoubling of efforts to help them, rather than threats and discrimination. As anyone who has ever been called fat or lazy knows, such labels, rather than acting as an incentive to reform, merely deepen the sufferer's sense of hopelessness and paralyse their willpower.

Mr Cameron's suggested welfare cuts hint at desperation as well as hard hearts, and one wonders who he will come for next. While billionaire tax evaders continue to live high on the hog, will benefits be taken from those who are ill because they were smokers, or disabled after playing rugby or taking up hang-gliding? Once the finger of blame is pointed at the needy, there is no limit to who might be targeted.

In a bid to whip up votes, the prime minister has said that paying benefits to those who won't help themselves is "not fair on taxpayers".

Well, I am happy for my taxes to help those unable to work. In fact, I would like them to get more support, not less. So what if we allow a few job-dodgers or fraudsters to go undetected? That's a small price to pay for a kinder, more tolerant and dignified society, where those with problems are not treated as pariahs, but as people, like everyone else.