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As if there weren't enough things you could catch from a new partner, now more and more people are finding that they're getting a new type of STD from their beloved.

It's silent, it's insidious and you usually don't know about it until it's too late. It's a 'sexually transmitted debt'. His debt, or her expensive little secret, can be a devastating blow to a new, or even a long-standing, relationship.

What are STDs?

Sexually transmitted debts are ones that are really your partner's but affect you. According to the relationship charity Relate, money is the biggest single cause of arguments in relationships. Suffering from your partner's debt would certainly put a major strain on even the best of relationships.

"People find it harder to talk about money in relationships than sex," says sex and relationships counsellor Christine Webber. "But you absolutely have to put all your cards on the table at the start and be honest with each other about money."

But fewer people than ever are being honest with their spouses about the money they have - or don't have. Research from the online bank Cahoot has shown that over 60 per cent of women in Britain have secret savings that they don't tell their partners about, very often because they don't trust his spending habits.

How can it affect you?

You are more affected by your partner's debts if you are married, but whether you are married or co-habiting, if you want to take out a mortgage together you will need to know whether your partner has a poor credit rating before you apply. You may not want to know too much about your partner's seedy history of unfaithful borrowing, but lenders will.

Many lenders have a weird tendency to reject applications for any type of loan if another lender has already rejected you for something else. So before you apply for a mortgage, or any type of loan together, you should make sure you know if your partner has been turned down for loans or, worse, if they have any County Court Judgements (CCJs) against them.

If they do have one, that will mean that most lenders will turn you both down immediately, which will make it harder to get a mortgage with someone else.

How can you protect yourself against it?

Noel Munro of the credit reference agency Equifax says that if you're worried about your partner's, or ex-partner's, bad debt affecting you, you should apply for your credit history to find out what is written about you.

"If you see something from your former partner that affects your credit rating, you can ask to put a 'notice of dissociation' on your file, which means that when lenders look at your file they don't see any of his records," says Noel.

Once you're married it's a good idea to have separate bank accounts, credit cards and savings accounts as well as a joint account for bills. This way, even though you will be liable to pay part of any joint account debts, at least you would have a chance of stopping them spending your money and, therefore, putting you both in a worse situation.

If you're living together and you want to buy a place, it's best to rent first before buying. Research shows that live-in relationships are even more likely to split up than married ones, so it's best not to commit yourself to the burden and complication of a mortgage too soon.

It's important to be aware that with joint loans or credit cards you are both liable for the debts. If your partner defaults on their share, the lender will come after you for it, so think very carefully before entering into a joint arrangement with your partner.

Jasmine Birtles is from Money Magpie - the website that gives you a richer life