If we are talking about just spreading the infection (virus), of coarse, higher concentration of the pathogen in environment would increase the chance of infection, it is just statistical probability (easier to cross the threshold). Also, higher number of pathogens in environment could "serve" a bigger community - more individuals exposed and thus, higher chance that pathogen will meet the individual who is more sensitive to it. Once, this "sensitive" individual got sick, pathogen could multiply and reach threshold level for the normal population. It's called epidemic. There are two sides in this equation: one is increase of the threshold (more pathogens needed to make bee sick) and another is to keep the concentration of the pathogen below established threshold. Later required constant "treatment" or as Mark graciously called it "medicate" to keep levels lower than threshold.