These graphic images of syphilis show the gruesome reality of what unprotected sex can lead to.

This 38-year-old man needed hospital treatment after developing painful, crusted skin ulcers all over his body after contracting the STI.

What started off as painless lumps developed into pus-oozing sores in two weeks, although it is likely he caught the infection months - or even years - earlier, doctors say. 

The pictures have been released by Chinese medics reporting in The Lancet medical journal to show the serious effects some strains of syphilis can have on the human body.

The man was treated at the No 1 Hospital of China Medical University in Shenyang, China. 

The doctors who treated him wrote in The Lancet: 'The man had a 1 month history of multiple painful skin ulcers on his face, trunk, and extremities.

'The eruptions had started [small]... and progressed to pustules and painful ulcers within 2 weeks.' 

As a result, the patient was first diagnosed with pyoderma gangrenosum, a rare skin condition that causes painful ulcers. 

But his symptoms worsened and after a week, he developed a fever, while being treated in hospital.

Further tests confirmed he was suffering from malignant syphilis, a rare strain of the STI.

The man reported having had sex with four women during the past year - but denied ever having sex with men.

Every year, 498 million people aged 15 to 49 are infected each year with chlamydia, gonorrhea, syphilis, or trichomoniasis, according to the World Health Organisation.

In the US, the number of cases of syphillis jumped 10 per cent between 2012 and 2013, with gay and bisexual men accounting for 75 per cent of the increase.

Similarly, recent Public Health England figures revealed cases of deadly syphilis jumped 33 per cent in total between 2013 and 2014, with a 47 per cent surge among gay men.

It was blamed on a failure of people to use condoms.

The three stages of syphilis begin with a highly infectious sore on the genitals, or sometimes around the mouth.

If somebody else comes into close contact with the sore, typically during sexual contact, they can also become infected. 

The sore lasts two to six weeks before disappearing, according to NHS Choices.

This may then develop into a skin rash and sore throat which can disappear within a few weeks.

A hidden phase with no symptoms can last for years but after this, syphilis can progress to its third and most dangerous stage - years after the original infection.

About a third of untreated cases develop into the late stages where the disease can damage your brain, nerves, eyes, heart, blood vessels, liver, bones and joints. 

If diagnosed early, syphilis can usually be treated with antibiotics.