How much body fluid required to actually transmit a sexually transmitted disease (STD)?

Question

While reading about sexually transmitted disease, I always wonder how much body fluid is actually required to transmit some diseases, specially in the case of other kinds of sex (masturbation, hand job, fingering and genital contact).

For example, this website list chances of getting STDs: Know your chances - Smart Sex Resource. Specially the table about other kinds of sex mentions that you can get gonorrhea and chlamydia from genital to genital contact, with sharing of body fluids. But exactly how much? Is there any literature on that?

I'm thinking in a scenario like: person A is fingering person B. Person B is infected with chlamydia/gonorrhea (easy to transmit). Person A has person's B fluid on his fingers/hands. Person A uses a towel to dry their hands a bit before engaging in "self" masturbation. What's the risk of person A getting infected?

I'm asking because anal/vaginal/oral sex without condom poses a clear risk, but sex isn't mechanical. People will kiss, finger and masturbate each other during sex and a tiny bit of fluid may be swapped.

Answer 1

To get even close to how much bacteria there are in vaginal discharge and penile discharge, we look at how are they diagnosed/detected laboratory wise.

Gonorrhea is caused by Neisseria gonorrhea a gram-negative diplococcus that is found in the discharge with pus seen in microscopy after gram staining. There are other microorganisms that are gram-negative diplococci; Examples of gram-negative diplococci are Neisseria spp., Moraxella catarrhalis, and Acinetobacter spp. Isolation of Neisseria is required by culturing it in a Thayer martin medium or by detecting its proteins by PCR.

In short, we can't exactly count how much bacteria there are in a droplet or an mL of vaginal/penile discharge since all other bacteria in there look the same under the microscope.

Another factor would be bacterial virulence and how competent an individual's immune system is. We cannot exactly determine at what rate the bacteria are reproducing and the competence of the immune system since both of these factors vary so much.

I can't seem to find any experimental study on deliberately exposing test subjects to Neisseria and which dose can cause an infection.

Source: Jawetz, melnick, adelberg; medical microbiology 26th ed. page 73, 289

The laboratory diagnosis of Neisseria gonorrhoeae Lai-King Ng, PhD and Irene E Martin, BSc

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2095009/