Symptoms are recognisable as an urge to become a member of a building society before it converts to a bank.
This week Birmingham Midshires members heard their society was to be taken over by Royal Bank of Scotland. The pounds 600m-plus price tag equals about pounds 600 for each of Midshires' 1.1 million policyholders, including about 300,000 speculators who joined the society in recent months for just this reason.
Many building societies hoping to keep out carpetbaggers have either pulled down the shutters or set high cash minimums of pounds 2,500 or more to open an account that confers membership rights.
One way to ensure a share in any payout, while earning a decent return on your investment, is to buy Permanent Interest Bearing Shares (Pibs) issued by many building societies as a means of raising money. Ownership of Pibs confers member status.
Pibs pay a fixed income and their prices have been going up. In part, this has been a response to the potential for free shares from future demutualisations. So how do they work, and are they a surefire route to windfall gains?
Building societies are prevented by law from borrowing in the way that banks can - from other banks.
This is because mutual status limits the liabilities that a society can enter into when borrowing and obliges it to protect members who have deposit accounts or mortgage loans. Pibs offer a convenient way for societies to get round the problem.
For example, Bradford & Bingley made one issue to raise pounds 60m, with a "coupon rate" of 13 per cent.
This means that the society raised pounds 60m by offering to pay a yield of 13 per cent gross, split between two annual pay dates.
With basic-rate tax deducted at source, the yield was cut to 9.75 per cent. Once issued, Pibs can be bought and sold like any other share. The price of Bradford & Bingley Pibs has gone up,reducing the current gross yield to around 7.99 per cent, which comes down to 6.07 per cent net for a basic-rate taxpayer.
The price of Pibs has generally tended to rise, reflecting the relatively high interest rates prevailing when most of them were issued five to six years ago.
As our table shows, gross returns are now between 7.6 and 7.99 per cent for all issues. Are they a good bargain?
Justin Urquart-Stewart, a director at Barclays Stockbrokers, warns: "The market for Pibs is linked to interest rates. Speculation [over conversion to bank status] may be driving their prices up, but if the wave of demutalisation slows down and interest rates rise, prices could suffer.
"Remember, when you buy Pibs you are really buying a stream of future income. If the price of Pibs falls, you may not get back the purchase price."
Of course, were the price of Pibs to fall, the income receivable from the shares would go up correspondingly.
Pibs are not subject to capital gains tax, but neither can they be placed in a PEP, the wrapper used to protect many investments from tax.
Income is not guaranteed, while payments can be suspended if the society decides that making them puts its solvency at risk. If the society is wound up then owners of Pibs come last on the list to be repaid, after depositors and other creditors.
Mr Urquart-Stewart concedes: "This is unlikely. The investment is low risk." He adds, however: "These are a dying breed of investment, never much traded, or very popular, and rather left out in the cold by corporate- bond PEPs. They represent another way into windfalls, but a lot depends on the pace of future demutualisation."
Not everyone agrees with this assessment. Last year JP Cairngorm Asset Management, a small Scottish fund management company, launched a Building Society Investment Trust to buy into Pibs and related bank bonds.
Chairman Ken Murray points out: "It's a matter of when you buy and how you do it. The trust has holdings in all society Pibs. We therefore manage a portfolio. This should reduce risk for the investor."
The Cairngorm trust is split into 10 subsidiary trusts, a structure designed to maximise the return from windfalls. Shares are currently trading at around pounds 9.40 for 10, with an estimated net asset value of between 113 and 115 pence per share, based on the current market for all stocks held, with a net yield of 4 per cent.
Mr Murray's strategic view is clear: "We expect demutualisation to continue as a trend."
Of course, predicting when a particular conversion will take place is impossible and societies are keen to dispel speculation.
Buying Pibs makes the purchaser a member of the society just like an account holder or borrower. This means that if the society demutualises, converting to bank status, all members may be due to participate in any windfall of shares or cash.
Most societies issuing Pibs give immediate membership to Pibs owners. Some may impose a minimum period on ownership as they do with account holders.
JP Cairngorm's trust hopes to benefit from this but Mr Murray accepts that windfalls are not guaranteed. The Building Society Act only makes provision for part of the society's funds or equivalent shares to be distributed among members, subject to rules of eligibility.
When demutualising, societies can therefore choose to distribute cash or shares. The law prevents cash windfalls to members of less than two years.
Neither are there any statutory rules on the qualifying period of membership for share windfalls. Although not likely, a society could backdate this to the date of first press speculation on demutualisation.
Moreover, the price of investing in Pibs can be at least as high as simply opening a speculators' account with a society.
In the case of Bradford & Bingley, minimum subscription is pounds 10,000, although for Britannia, Coventry and Skipton - all societies in the frame over possible conversion - minimum levels start at pounds 1,000.
The only certainty of investing in Pibs is the future income they will pay. Those suffering windfall fever should take an aspirin and find qualified advice from a stockbroker.
Barclays Stockbrokers, which specialises in Pibs, can be contacted on 0345 777 4000.