There's a well-established rule among investors: when a bad thing happens to one company, get rid of the shares of companies in the same position, at least until the dust settles.

When a car maker discovers a fault in its leading model, for example, all car firm shares fall, as investors fear its rival may have similar problems.

Little wonder, then, that Enron - the biggest and nastiest piece of corporate news in recent memory - has caused ripples among a host of other firms.

But the extent of the fall-out has caught many by surprise: after a modest start, "Enronitis" has been detected among the bluest of blue chips, and in markets all over the world.

After a difficult week on Wall Street, it seems to have halted, even threatened to reverse, the post-11 September rebound.

First snuffles...

So how long will the contagion last?

Like all epidemics, Enronitis started quietly.

The first jitters were seen in the shares of firms that were either similar to Enron in their business model, or had direct dealings with the failed energy trader.

Shares in Dynegy, the trading company that came close to acquiring Enron last year, have lost more than half their value since the beginning of December, for example.

On a more modest scale, International Power, a British utility that has been sniffing around some of Enron's electricity assets, has seen its shares fall by 20% since the US investigation into Enron began last month.

Even a solid blue-chip firm like investment bank JP Morgan Chase has suffered a 25% share slide this year, as fears grew of its likely exposure to Enron debt.

... become outright 'flu

All these ripples are perfectly comprehensible.

But the difference between Enronitis and the usual post-news reaction is the speed with which the pool of afflicted firms widened.

After hammering companies with direct links to Enron, bearish investors have turned their fire on any firm suspected of using similarly deceptive accounting practices.

Since Enron-style "aggressive accounting" - using book-keeper's tricks to massage earnings - has been highly fashionable in recent years, the list of targets is long.

In the past few weeks, the main casualties have included:

• US conglomerate Tyco, once a highly fashionable firm, was hit by a lawsuit from shareholders, alleging that its network of off-balance-sheet subsidiaries concealed all manner of horrors. The firm had made over 200 acquisitions over the past three years.
• Telecoms and internet giant WorldCom has had to reassure investors that it was not on the point of bankruptcy. The firm was forced to take a charge of up to $20bn thanks to a change in accounting rules, and its shares have lost more than half their value this year.
• Cable firm Global Crossing, another client of auditor Andersen, went bankrupt after suffering a collapse in investor confidence. The company's Bermuda domicile, determinedly optimistic forecasting, and close relationships with politicians are now seen as danger signals, and the FBI has launched a probe into the firm.
• Energy giant Williams admitted it had no idea about how to account for more than $2bn in debts owed by a former subsidiary. Its shares have lost one-quarter of their value since December.
• Shares in Enterasys Networks dropped by 61% in one day, after the hi-tech firm admitted that the Securities and Exchange Commission is investigating it and unidentified "affiliated companies" .

Contagion

Most of these companies - and their many counterparts - have something in common.

Even during their fashionable periods, these firms were high-risk, high-return investments, willing to do whatever it took to dominate their dynamic, highly competitive markets.

What is so notable about Enronitis is the way that it has also started to afflict some of the most venerable names in business on both sides of the Atlantic.

• General Electric, the world's biggest company and the darling of investors for decades, has come under fire for its accounting practices. Although no one alleges any form of malpractice, the company is seen as too sprawling, complicated and diverse - precisely the sort of faults detected at Tyco.
• British engineering giant Rolls Royce has been the subject of newspaper speculation - which it firmly denies - that its shareholders were concerned over accounting irregularities.
• UK hi-tech engineer Invensys has seen its shares tumble by about 20% in the past couple of weeks, as investors speculated that its debts could force it into drastic action.
• Similar, although larger, worries swirl around indebted German media giant Kirch - something that has weighed on the share prices of Germany's biggest banks.

Glasses half empty

In themselves, none of these jitters is particularly unusual.

But the fact that they have emerged at all, and all at the same time, is a function of the nervousness that Enron has produced.

"People are looking for reasons to be scared," one broker told BBC News Online.

"Six months ago, investors always saw the glass as half full; these days, it is always half empty."

But opinions are still divided on whether the gloom will persist.

Pessimists argue that there are plenty more firms to investigate, and that the roll-call of unearthed horrors has only just begun.

No pain, no gain

But there is also an optimistic view.

The current hoo-ha over Enron and the role of the accountancy profession may well result in new, tighter rules for corporate reporting.

At very least, it should encourage investors to look a little harder at the companies they own.

And reputable firms - Wal-Mart and Boeing are two recent examples - could seize the opportunity to make a public display of good behaviour.

It is possible that a stronger, safer and more reliable stock market might emerge out of the current mess.

But if the Enron affair teaches us anything, it's that making any sort of prediction about the stock market is next to impossible.