Know how to use "fewer" and "less"? Find out.
Old English mid "with, in conjunction with, in company with, together with, among," from Proto-Germanic *medjaz (cf. Old Norse miðr, Old Saxon middi, Old Frisian midde, Old High German mitti, Gothic midjis "mid, middle"), from PIE *medhyo- "middle" (see medial (adj.)). Now surviving in English only as a prefix (mid-air, midstream, etc.); as a preposition it often is a shortened form of amid (cf. midshipman).
minimal infecting dose