DOTS. So it is, first of all, the duration of infection. How long is someone spreading contagion for? But it’s also the opportunities for spread during that period. How many interactions they have, how many others might they expose? It’s also the transmission probability during one of those interactions.
Epidemiology is full of such examples. For instance, the link between smoking and cancer. You can’t run an experiment and get a bunch of people to take up smoking and see what happens. You have to piece together from observation what you think might be going on and ascertain the level of threat you’re dealing with. Ultimately, I think that’s where epidemiological thinking can be very useful.
People who claim that epidemiologists and doctors disagree with each other and give out conflicting advice are partially right…and completely missing the point. There should be disagreements because this is a *NOVEL* virus…as in we’ve never dealt with this before!
There are also plenty of advice and guidelines that the vast majority of healthcare experts and scientists do agree on, and it’s easy to follow the evolution of all that advice as they learn more and more about what we’re facing.