If I touch my cell phone after working on my customer’s nasty, crusty, dirty computer, but before I wash my hands, will my phone immediately be infected? Do viruses and bacteria stay in the general area of the infected surface or will they eventually cover the whole thing?
Viruses need a host organism in order to replicate. if you deposit X viruses on your cell phone, you will never see more than X viruses on your cell phone. Depending on exactly what the virus is (common cold, flu, HIV, Ebola, etc.), it may or may not remain viable on your cell phone for long.
Yes, and on the sorts of surfaces in question it will not be very much different for bacteria. They can reproduce by themselves, but they need some nutrition to do so, and they won’t find much, if any on the surface of a phone (maybe a little bit in whatever muck they got deposited along with, but maybe not even that depending on the type of bacteria and the type of muck). Furthermore, most of teh bacteria capable of infecting a human body need the level of warmth they find in a body in order to reproduce, so even if they find some food on the surface of your phone, it is probably too cold for them. Also, many bacterial infections are passed on by bacterial spores, which will be inert until they get into a suitable environment for the bacteria to grow, like your body. Spores will, however, survive for a long time through a wide range of conditions.
So, the general answer to your question is, it happens straight away, but it does not generally then get worse over time. (And often gets better: that is, many things won’t survive for long on a non-living surface. But some will, unfortunately.)