It is not any one cigarette or one extra drink that is ruinous to the health. The damage is done over the years, almost imperceptibly. Grave threats to the health of democracy can also accrue so incrementally that they draw little attention.
A committee of peers diagnose one such danger today in a report on the steady creep of surveillance. The charge of hysteria is routinely used to sweep aside such talk when it comes from crusading journalists and pressure groups.
The Lords constitutional affairs committee, however, cannot be dismissed the same way. A more dignified band of dignitaries would be hard to imagine - it includes a former attorney general who is a conservative champion of that antiquated role, a Tory expert on the constitution, and a founder of that force of militant moderation that was called the SDP.
Their insistence that mundane data collection "risks undermining the fundamental relationship between the state and the citizen" may be dramatic, but it is rooted in careful argument. Privacy is not only a precondition to a life of any quality, it is part of the meaning of liberty. The rule of law in Britain is not codified in a constitution, but underpinned by shared support for the twin ideals of executive restraint and individual freedom.
Under the gaze of 4 million CCTV cameras, and in the face of the burgeoning electronic tabs being kept on citizens, both ideals are strained. Bit by bit the state - and private firms - cease to believe that the courtroom is the place to hold individuals to account, and instead grow used to monitoring them in all sorts of contexts in the name of convenience. Bit by bit, meanwhile, individuals learn to live with the ubiquitous prying eye.
Technical change rather than political choice explains much of this drift. As collecting information gets cheaper and easier, it starts being collated in ways that no one would have dreamed up in the past. The committee does not dispute that this can bring gains, from cracking crimes to ensuring patients receive consistent treatment. As with complex derivatives in the City, however, the great problem has been that regulation has not kept pace with innovation.
The peers suggest sensible steps to redress the balance - for instance, a new requirement on public bodies and firms to encrypt the personal data they hold to cut the risk of it falling into the wrong hands. An independent review of the proclaimed but largely unproven benefits of CCTV could help ensure it is used only where it really does make a difference. Automatic assessment of what government announcements mean for privacy - something already required for race equality and red tape - would build a prompt into the system so that Whitehall would get into the habit of considering the issue, a prompt that could help to turn the tide.
Failure to think is not always the problem - sometimes it is bad deliberate decisions. The peers rightly insist that it is just not acceptable for the state to hang on to the DNA of individuals never convicted of a crime, purely on the arbitrary basis that they once came under suspicion. Strasbourg recently said the same thing, in a ruling that must now be given effect. The wide powers to snoop that council officers have been handed need to be trimmed. Judicial oversight is part of the answer; another part is making sure the powers are used proportionately. Following someone suspected of a violent crime is one thing; following a parent suspected of fibbing about their address to get their child into the right school is quite another.
One of the few shortcomings of the Lords report is its silence on those threats to privacy that ministers are currently pushing, notably the super-database on mobile communications. That silence may be the price for achieving all-party consensus. Even after that price has been paid, however, the committee has done invaluable work. It has nailed the age-old lie on surveillance - by asserting that those with nothing to hide can still have a great deal to fear.