18 June 2014 Last updated at 16:48

The silent spread of sexually transmitted infections (STIs)

An electron microscope image of a cell infected by the bacteria of the STI chlamydia trachomatis

A human cell infected by the bacteria of the STI chlamydia (the green substance in the centre).

Nearly half a million people in the UK are diagnosed with a sexually transmitted infection (STI) every year. However, the number of actual cases is likely to be far higher as STIs can often be passed on unwittingly.

How many STIs are diagnosed?

In England alone between 2010 and 2011 there were 426,827 new infections in a 12-month period which included:

Chlamydia: 186,196. Most of these were people aged 24 or younger
Genital herpes: 31,154
Genital warts: 76,071
Gonorrhoea: 20,965. The highest rates of gonorrhoea are seen in women aged 16-19 and men aged 20-24
HIV: 6,280
Infectious syphilis: 2,915

Source: HPA - based on diagnoses between 2010 and 2011 in sexual health clinics in England

Sexually transmitted infections are spread from person to person through sexual contact. These include more than 30 different bacteria, viruses or parasites. Others, such as HIV and syphilis, can then be passed on even further. This can occur either through blood transfusions or tissue transplants, or from a mother carrying the infection to her child during pregnancy and childbirth.

Left untreated, STIs can cause serious complications such as infertility and impotence and some may even prove fatal.

Recorded cases of infections are steadily rising. Whilst there is greater awareness and subsequent testing for the conditions, this is not the only reason. STIs are easily spread and maybe passed on before a person realises they have the disease. Younger adults are at greater risk as they are more likely to have unsafe sex with multiple sexual partners.

Some infections, such as chlamydia and HIV, may not cause any obvious symptoms. Conditions like syphilis can cause painless sores that can easily be missed. The virus that causes genital herpes also usually has few initial symptoms, so 80% of people carrying it don't know they've been infected.

What will happen if bacteria like Gonorrhoea become resistant?

Other STIs don't cause symptoms straight away but can be spread during this window of time - it could take months before someone shows symptoms of genital warts, for example.

The virus that causes genital herpes also usually has few initial symptoms, so 80% of people carrying it don't know they've been infected.

Precautions such as wearing a condom are an important way to reduce the risk of STI's but still can't prevent the spread of parasitic infections like pubic lice.

STI	WHAT ARE THE RISKS?
Source: NHS choices
Chlamydia: A bacterial infection that is found in sexual fluids	Most people who have chlamydia don't notice any symptoms and won't know they have the infection. However, if left untreated women can develop pelvic inflammatory disease, which can lead to pelvic pain, infertility or an ectopic pregnancy. It can also lead to an infection of the womb, ovaries or fallopian tubes, which can cause infertility. It has been linked to fertility problems in men. It can be successfully treated with antibiotics
Genital warts: A skin infection caused by types of the human papilloma virus (HPV)	Causes fleshy growths that appear around the genitals from three months after infection. The growths are usually painless but can be unsightly. The types of HPV that cause warts do not usually cause cell changes that develop into cancer. Warts can be successfully treated at a sexual health clinic, also known as a GUM clinic
Genital herpes: An infection caused by the herpes simplex virus	Can cause painful blisters on the genitals. It is a long-term condition because the virus can lie dormant in the body and then become active again. It recurs an average four or five times in the first two years after infection. Flare-ups reduce over time. Symptoms can be controlled with anti-viral drugs
Gonorrhoea: A bacterial infection that is found in sexual fluids	Like chlamydia, gonorrhoea can cause pelvic inflammatory disease in women if left untreated. In men it can lead to a painful infection in the testicles and prostate gland, increasing the risk of reduced fertility. It can be treated with antibiotics though some strains are becoming resistant
Syphilis: A bacterial infection passed on via infected sores	Syphilis causes painless infectious sores that lasts up to six weeks. Skin rash and sore throat then develop. If left to progress, syphilis can cause serious conditions such as stroke, paralysis, blindness or death. It can be successfully treated with antibiotics if caught early
HIV/AIDS: A viral infection that attacks the immune system. The final stage is AIDS	The human immunodeficiency virus (HIV) attacks the immune system, and weakens the body's ability to fight infections and disease. The final stage, when the body can no longer fight life-threatening infections, is AIDS. There's currently no cure for HIV but there are treatments that enable most people with the virus to live a long and healthy life
Pubic lice: Tiny parasitic insects that live in body hair and are passed on by close contact	The 2mm-long blood-sucking lice cause itching and inflammation in affected areas. It can take weeks for symptoms to develop. They are not the same as head lice and aren't linked to poor personal hygiene. Can be successfully treated with medicated ointments