The terms microbe and microorganism are the same, and are often used interchangeably. M-W dictionary defines microbe as "microorganism, germ." While microorganism is defined as "an organism of microscopic or ultramicroscopic size." Viruses can be called either and it usually depends on the preference of the writer for which term they will use. Microorganism can get a bit bulky to write repeatedly and so the switch to microbe can occur. Otherwise there is no real scientific distinction.
