Let's distinguish between bacteria and viruses, for instance.
You have different temperature requirements. You have different requirements. You have viruses in special culture flasks that you grow them in. For instance, if you were to encounter HIV, which is a virus, in its natural environment, you would have a very low concentration of those viruses within the blood of a patient. However, if you take HIV, impart it into cells and grow those cells, you would have a much higher concentration of those particular viruses within the same volume. As such, if you were to expose yourself to one drop of human blood infected with HIV, that would be quite different from the number of actual organisms within one drop of tissue culture fluid.
The same goes for bacteria. If you were to grow them, you would have much more per volume than you would have in the natural environment.