Yes, the type of virus is all important and then there is the issue of how the person is exposed to the virus, there are lots of barriers in place. The virus then has to somehow navigate through the host, which is generally a very hostile environment and which contains a variety of specific and non specific chemical agents that can be very damaging to the virus.
The presence of high levels of specific antibodies makes the possibility of establishing an infection very remote indeed. It must find a cell to which it can gain entry, usually this means the cell has to have specific receptors on its surface that the virus can exploit, it must then co-opt the cells machinery to be transported to the nucleus, while avoiding the inbuilt checking mechanisms. It has to get through another barrier protecting the nucleus and inject its own genetic material.
Cells do have their own quality control checks, and if the viruses are detected the cell can start to express specific markers on its surface that act as red flags to the immune system. If these cells are detected the cell will be destroyed along with its pathogens, the virus has to survive long enough for the new genetic program to produce millions of new virus particles which will be released to infect more cells, this process usually kills the cell.
All the time this is happening the bodies defences become increasingly aware that there is a problem, which it begins to investigate, this response ramps up in response to the perceived threat. We only start to develop symptoms when the virus has become well established and self sustaining and many of the symptoms occur as a result of the bodies defences becoming increasingly aggressive. Many of these defences are also highly variable in the human population.
Many viruses are rather slopping in their reproduction, they rely on numbers rather than quality, a large number of the viruses will in fact be incapable of causing infection. The ones that are have to run an incredible gauntlet of defences and then have to reproduce fast enough to overcome the first defences and establish an infection before the adaptive immune system develops specific antibodies.
So with all of that the possibility of a single virus particle causing an infection is about as likely as them developing microprocessor technology, its well established that in virtually any infection a specific dose of the pathogen is needed, this dose varies with the pathogen, the method of infection and the host.
The links don't describe a single virus causing disease, they describe a single genetic variant of the pathogen becoming dominant, this is entirely predictable, the genetic variants in a population most able to establish infection will be the ones to succeed.
Sorry I just found this which is much more specific to Covid 19
https://www.sciencemediacentre.org/expert-reaction-to-questions-about-covid-19-and-viral-load/
