STD Pathophysiology

Sexually transmitted diseases (STDs) are transmitted via the mucous membranes of the vagina, penis, urethra or rectum, during sexual contact with an infected individual. Transmission can also occur via membranes of the throat, respiratory tract, mouth and eyes.

The mucous membranes are thinner than skin and allow microbes to cross them into the body. In addition, minor breaks in the skin and mucous membranes due to abrasions or cuts further increase the risk of infection. The microbes are often present in fluids secreted from the penis, vagina, saliva, faeces, urine and sweat. Sometimes, only a small amount of the microbes can lead to transmission of an STD.

Any sexual contact including oral sex and deep kissing can lead to the transmission of certain infections, although the concentration of the microbes is often lower in saliva than in the genital fluids. Non-sexual contact such as hugging and shaking hands is not usually responsible for transmitting infection.

Causative agents of STDs

The causative agents of STDs include bacteria, fungi, protozoa and viruses.

Some of the more common types of infection include:

Bacterial infections


This is one of the most common STDs. There are often no symptoms of the infection but symptoms that may manifest in women include pain, vaginal discharge, bleeding and pain during or after sexual intercourse and a burning sensation while urinating. In men, it can cause a white, cloudy or thin, watery discharge from the tip of the penis along with pain during urination or sex. If left untreated, this infection may lead to infertility.


This is a bacterial infection that can be transmitted through any sort of unprotected sex. While often symptomless, in women, the infection may lead to pain, vaginal discharge (watery, yellowish or green), bleeding and pain during or after sexual intercourse and a burning sensation while urinating. In men, it can cause a watery, yellowish or green discharge from the tip of the penis along with pain during urination or sex. The infection may also affect the throat, eyes or rectum.


This is a bacterial infection that has several phases of development. In the first stage, highly infectious sores on the genitalia or mouth develop and persist for up to 6 weeks. During the next stage, a rash, fever, and hair loss may occur but subside in a few weeks. In the late stage or tertiary stage, which may occur years after the first two phases, there may be serious destructive infection of the nervous system that can lead to paralysis and blindness.

Viral infections


Herpes is caused by the herpes simplex virus. The infection causes painful, itchy blisters or sores to develop around the genital area that make sexual intercourse and urination painful. The blisters may subside and remain absent for a while, before flaring up again.

Viral warts

These are painless but itchy warts that develop across the genitalia as a result of human papilloma virus (HPV) infection. Certain strains of HPV infection can eventually lead to cervical cancer but most infections pass without any symptoms.

HIV infection

The HIV virus attacks and damages the body's immune system. The symptoms may take a long time to appear, with affected individuals often unaware they have the infection and therefore at risk of unknowingly passing the virus onto other people through unprotected sex. Other modes of transmission include needle sharing, blood transfusion, or mother-to-baby transmission during childbirth.

Parasitic infestations

Trichomonas vaginalis

This is caused by a parasite that is transmitted during unprotected sex. In women, the infection leads to pain and a frothy, yellow or watery vaginal discharge. In men, there may be a burning pain or no symptoms at all.

Lice or crabs

Pubic lice can also spread via unprotected sexual contact. These lice can also live in underarm hair, body hair and beards.


This is caused by mites that burrow into the skin and can be picked up through sexual or intimate contact or from using clothing and towels in which mites are present. Scabies causes intense itching in affected areas, particularly at night.

Further Reading

Last Updated: Jun 21, 2023

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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