Sexually Transmitted Infections

What are sexually transmitted infections?

Reading time: 4 min

Sexually transmitted infections or diseases (STIs/STDs) are illnesses caused by microorganisms transmitted between people, primarily through skin/mucous membrane–skin/mucous membrane contact or through fluid exchange during sexual intercourse.

STIs are provoked by different types of microorganisms: yeasts, bacteria, viruses and parasites.

Some STIs mainly affect the genitals but they may also affect other areas such as the anal and perianal region or the mouth. Furthermore, there are other STIs that can spread and affect other organs.

It is important to realise that several of these infections can go unnoticed, particularly at the onset, and some of them may become chronic or have permanent consequences.

Are sexually transmitted infections very common?

In recent years, the overall incidence of sexually transmitted infections in our society has increased due to several factors:

Sex between two men

A decline in the use of protective measures during sexual intercourse. Modern treatments have increased the life expectancy for HIV patients and this has reduced the fear of becoming infected; consequently the general population tend to take less precautions.

Pareja sexual

An increase in the use of new technologies to find sexual partners (mobile applications, chats, etc.). These technologies facilitate sexual activity with unknown or anonymous partners and the possibility of having sex with several partners simultaneously.


An increase in drug consumption. Sexual intercourse under the effects of certain drugs hampers one’s control over the protective measures required to prevent infection.

travel, ariplane

An increase in the mobility of people from different backgrounds with no access to diagnosis, treatment or preventative measures.

couple with travel luggage

An increase in sexual tourism.

How many people does Sexually Transmitted Infections affect?


Human papillomavirus (HPV) is the most common STI worldwide, and also in Spain. Estimates indicate that over 40% of adults in the US are infected with papillomavirus, although in most cases the infection goes undetected and clears spontaneously within a few months. The number of HPV diagnoses has remained stable in recent years.

Genital and oral herpes.

These infections have also increased in recent years. The infection commonly passes undiagnosed and therefore unregistered which makes data unreliable. It is calculated that between 10% and 20% of the population in the US are infected with type 2 herpes virus (the strain most commonly found affecting the genitals). In Spain, recent studies estimated there are 46.3 cases in every 100,000 inhabitants.


There were 66,413 cases of gonococcal infection diagnosed across Europe in 2014. That represents an overall rate of 20 new cases per 100,000 inhabitants. Almost half (44%) of those diagnosed were men who have sex with men. The number of new cases had increased by 25% in all patient groups compared to 2013.


Genital Chlamydia trachomatis is the most common sexually transmitted infection throughout Europe. As many as 396,128 new cases were reported in 2014 at a rate of 187 cases per 100,000 inhabitants. Chlamydia often courses without presenting any symptoms, so it is believed that the number of cases could be much higher. Two thirds (68%) of all cases are reported in people aged under 25 years, most of whom are women.


The incidence of syphilis has been on the increase in Europe since 2010. As many as 24,541 new cases were reported in 2014 at a rate of approximately 5 in every 100,000 inhabitants. Six times more men have syphilis than women. The main group affected (almost two thirds of cases, 63%) are men who have sex with men and the majority of cases are diagnosed in people aged over 25 years.


Almost 30,000 people were diagnosed with HIV / AIDS infection across Europe in 2015, that is a rate of approximately 6 cases per 100,000 inhabitants. The rate of HIV incidence in Spain is estimated to be approximately 9 cases in every 100,000 inhabitants (2015 data). Most new cases are diagnosed in men (85.9%). The most prevalent form of transmission occurs between men who have sex with men (roughly 50% of all cases), followed by sexual transmission in heterosexual couples (25.4%) and the spread among people who inject drugs (2.8%). Practically half of all new cases are diagnosed late.

Lymphogranuloma venereum

The European Union registered 1,416 new diagnoses of lymphogranuloma venereum in 2014. Virtually all cases involved men who have sex with men; 87% of patients were also HIV positive. The number of new cases in 2014 represented a 32% increase on 2013. Several European countries do not keep a register for this infection and so it is considered very likely that the actual number of cases is higher.

Hepatitis B

Hepatitis B is a viral liver infection that can give rise to either an acute problem or a chronic disease. Globally, calculations indicate it affects 5 in every 100 people, but this rate varies according to the country. It is believed that between 2 and 8 in every 100 inhabitants are infected in Spain.

Hepatitis C

As with hepatitis B, hepatitis C can develop into a chronic disease. There are estimated to be between 130 and 150 million people worldwide with chronic hepatitis C infection. Roughly 1–3 individuals per 100 have been in contact with this disease in Spain. The sexual transmission of this infection has increased in recent years and it mainly affects men who have sex with men.

Hepatitis A

Due to its transmission route (faecal-oral), yet without being a “classic” STI, microepidemic outbreaks of hepatitis A have been observed among the population of men who have sex with men.

Substantiated information by:

Mercè Alsina
José Luis Blanco
Núria Borrell
Irene Fuertes

Published: 20 February 2018
Updated: 20 February 2018


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