Sexually Transmitted Infections risk factors

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Sexually transmitted infections (STIs) are primarily transmitted during penetrative sexual intercourse (whether it is vaginal, anal, or due to oral sex). What is more, some STIs can also be transmitted through sexual contact without penetration.

Sex between two men

Anal penetration. Entails a greater risk of catching certain infections (e.g., HIV infection) than vaginal penetration.

A person with an apple and a banana representing oral sex.

Oral sex. Many STIs (herpes virus, genital warts, gonorrhoea, chlamydia, syphilis) can be transmitted during relations based exclusively on oral sex, even though the risk appears to be less than the risk posed by anal or vaginal penetration.

Sex toy, vibrator

Sharing toys or material used during sexual intercourse also implies a risk of transmitting certain STIs.

Two people kissing

Kissing on the mouth or intimate contact (caressing, massages, mutual masturbation, etc.) carry a much lower risk of STI transmission than penetrative sexual activities and they do not present any risk with respect to infections such as HIV and hepatitis B.

Who is at risk of catching a Sexually Transmitted Infection?

Anyone who is sexually active is exposed to the possibility of infection, particularly if they do not use appropriate preventive measures. The fact that some infections are transmitted by skin/mucous membrane–skin/mucous membrane contact makes it difficult to practice fully protected sex. Furthermore, individuals who were sexually active in the past can present complications due to previously acquired infections that have gone undiagnosed or untreated. 

How are Sexually Transmitted Infections transmitted?


HIV, hepatitis A, hepatitis B, gonorrhoea (gonococcal infection) and genital Chlamydia infections. These are transmitted through the exchange of bodily fluids such as semen, urethral secretions or vaginal secretions. Gonorrhoea and Chlamydia can also be transmitted through exclusively oral penetration.

Liver and Hepatitis C virus

Hepatitis C. This strain of hepatitis is mainly transmitted via routes other than sexual intercourse; however, sexual transmission of hepatitis C has increased in recent years and it primarily affects men who have sex with men, suggesting that anal penetration is associated with an increased risk.  The consumption of drugs via injections (sharing syringes), as in the case of “slamming”, or even by other routes (e.g. nasal in the case of cocaine when sharing “snorters” or “straws”) also appear to be important routes of hepatitis C virus infection.


Genital/oral herpes, genital warts or condylomas and syphilis. These are transmitted through direct contact with the areas of skin or mucous membranes affected by the infection. They are transmitted very easily in the absence of a barrier method.

Penis and crabs

Scabies and pediculosis pubis (crabs). These can be transmitted via intimate bodily contact or infected clothing.

Some of these infections can be transmitted by means other than sexual intercourse:

Hand with a cut on a finger and a droplet falling down

HIV, hepatitis B and hepatitis C. These infections can also be transmitted through the blood of an infected person, by sharing illicit drug or pharmaceutical injection material, or via tattoos or piercings performed with unsterilised material.

Most sexually transmitted infections can be caught more than once, the individual just needs to be exposed to the given microorganism again, because the body’s immune system does not create adequate defences against these diseases.

It must be taken into account that many infected individuals are unaware of their condition. In addition, these infections follow a chain of transmission, in other words, one infected person can infect several others and each of them may cause new infections.

Substantiated information by:

Irene Fuertes De Vega
José Luis Blanco Arévalo
Mercè Alsina Gibert
Núria Borrell Iragari

Published: 20 February 2018
Updated: 20 February 2018

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