Die Hard said:
So how does the transmission of an STD work, anyway? I mean in detail... Like, if I put my finger in a woman's "pocket", nothing happens but when I put my "rod" in there, it transmits the virus. Why's that? I know the virus is in the blood but my "rod" isn't bleeding, is it? Basically, my "rod" and her "pocket" are having skin-on-skin contact, so what part of my "rod" contains and transmits the virus and what part of her "pocket" will absorb the virus? How does this work, exactly?
The transmission of a pathogen that causes an STI depends on what physiological/cellular machinery is required for the pathogen to attach to and infect the host.
If you take HIV for example, HIV is a virus. A virus, isn't really "100% alive", that is it only exhibits significant metabolism when it has infected a host cell. By themselves, almost all viruses are just particles of protein, DNA or RNA and perhaps a viral lipid envelop. They have far less "parts" than a true living cell. Viruses invade a host cell and subvert host cell machinery to make viral proteins, assemble viral particles, and eventually lyse and kill the cell resulting in release of many more virus particles.
The HIV virus (and other viruses) uses markers/molecules on the cell surface to attach to a cell, and inject it's nucleic acids into the cell to try and hijack the cell's interior machinery for the production of more viruses. In the case of HIV, the molecule is called CD4. This is found on helper T-cells (cells that coordinate immune response) and macrophages (basically the immunological equivalent of a battle tank).
In many cases even if the cell is successfully invaded, the virus can lie dormant for many years as a part of the host cell's genome.
Getting back to the original question about HOW infections occur, if the offending pathogen is a bacteria, say
N. gonorhea , it has molecular structures on the cell surface that are designed to attach to the host's epithelial cell layer. The host can subsequently become colonized and infected if the immune system isn't able to take care of the problem quickly enough. Keep in mind however, bacteria, unlike viruses are totally alive... Some STI bacteria such as
T. pallidum, the causative bug of siphilis actually can only stay alive in cells. It all depends on the bug that we're dealing with.
Infections occur when infective particles/cells, whether they bacteria or viruses, are present in fluids exchanged between partners. There must be a sufficient number of bugs present for successful attachment, colonization and infection. Hope this answers your question...