Three things you'd hate to hear while fellating someone for the first time: "That's not just a blister," "Watch your teeth – I'm positive!" or "Don't worry, even Pammy Anderson has hep C . . ." Strong sexual feelings may lead to an urgent desire to act, but nowadays things can get tricky. Telling someone you're about to sleep with that you have a sexually transmitted infection is a vital aspect of ensuring the act will be consensual – but it needs to be timely and appropriately handled. In a new social era, this is one of the unspoken rules of modern manners – and as such, it is still evolving.

When should you tell?

Sexual etiquette as our parents understood it is no longer relevant, and owning up to having an STI involves breaking one of the earlier cardinal rules of "nice behaviour" – bringing up something highly unpleasant. But ethicists advocate initiating that vital conversation the moment you think sex might be in the cards (long before you're in bed together), for instance try saying: "I'd love to be intimate with you, but first you ought to know that I have hepatitis C/herpes/am HIV-positive." [See footnote.]

Disclosing this information to a potential sexual partner means they can give informed consent, but some people think they've done their duty if they stop lovemaking just short of intercourse. In fact, unprotected oral sex carries certain risks – and so does kissing ("kiss and tell" should probably be the other way round). Leaving disclosure to the last minute is inappropriate for many reasons. It's sensible and fair to leave the other person some processing time. Asking someone who's already naked: "By the way, I'm positive . . . got a condom?" is just plain rude. Safety aside, allowing one's partner the opportunity to withdraw gracefully is a nicety I believe anyone with an STI should practice.

What should you say?

Why not try something along the lines of: "Hey, are we ready to disclose our deepest, darkest STIs?" (humour always helps), or "Given the animal nature of my feelings for you, I should warn you about something."

How much information is too much information?

Ideally, it should be a discussion where questions are welcomed, but is it wise to sound clinical? "For the record, I have herpes – which I control with Zovirax. I'm asymptomatic at the moment, but there's always a risk . . ." Mmm . . . sexy! But that certainly provides clear information, and signals caring for the other person. It takes practice and forethought to find a relaxed way to communicate all necessary information while maintaining one's seductive edge; the phrase "exchange of bodily fluids" is probably best avoided.

When should you ask?

Just as it's not fair to delay or withhold knowledge of one's sexual health from a prospective partner, it can also be said that it's remarkably stupid of the partner to avoid the question. All parties should take responsibility for the safety of a sexual act, and in my view that means always assuming that the other person does have an STI and acting accordingly. Everyone needs to be aware that, even with precautions, one is never 100% safe. And that people lie.

Then again, a person can be an asymptomatic carrier of an illness such as chlamydia without knowing it. Some people are woefully ignorant and fail to recognise obvious symptoms, while others are in denial about their infection. So telling someone you have an STI is actually less important than protecting yourself – and a partner – through safer sexual practices. I say, always assume the worst.

How will this change things?

Many people think being safe limits them erotically, but this is quite incorrect. We all have our own ideas about the level of risk we're prepared to take – whether cycling in a city, smoking, or eating donuts – but if you sit down and compose two extensive lists, headed: "Things I'd be prepared to do with someone I knew for sure was STI-free", and "Things I'd be prepared to do with someone who definitely has an STI" you'll find that – providing you allow for creative sexual behaviour – there's usually very little difference (often just "use condom for any form of penetration").

Eroticism extends way beyond basic friction and adapting one's sexual styles to those that will neither pose a threat to a partner nor put one's self at risk is an excellent strategy. Without the emphasis on straight-for-the-money penetration, working around an STI can even lead to more creative, more exciting sex. And having a mindset of always assuming your partner is a carrier can actually make you a better lover.

• This footnote was appended on 22 June 2011. After publication of this article, The Hepatitis C Trust contacted us to say that as hepatitis C is a blood borne virus, it only proves a risk during intercourse where blood-to-blood contact occurs. A 10-year study, published in 2004, of 895 heterosexual monogamous partners to hepatitis C infected individuals concluded "a risk of sexual transmission of HCV within heterosexual monogamous couples is extremely low or even null. No general recommendations for condom use seem required for individuals in monogamous partnerships with HCV-infected partners".