As an interested layperson (having small kids) I was told and read that some viruses are extremely infectious. E.g., very few virus particles of Norovirus (some say less than five) can cause an infection.
Now this implies that there is a threshold for an infection that is greater than zero for any virus (the actual numbers obviously vary a lot). I hypothesize that many people come in contact with a circulating virus but stay below that threshold. If this was true, would such people develop antibodies and a lasting immunity for viruses that our immune system can acquire one? When testing for antibodies during an ongoing epidemic would one then see a "wavefront" of immunity around the actually infected population?