germ: [17] As its close relatives germane and germinate [17] suggest, germ has more to do etymologically with ‘sprouting’ and ‘coming to life’ than with ‘disease’. It comes via Old French germe from Latin germen ‘sprout, offshoot’, which may go back ultimately to the Indo- European base *gen- ‘produce’ (source of English gene, generate, genitive, etc).
The meaning ‘sprout, from which new life develops’ persisted into English (and still occurs in such contexts as wheatgerm – and indeed in metaphorical expressions like ‘the germ of an idea’). Then at the beginning of the 19th century it began to be used to put into words the idea of a ‘seed’ from which a disease grew: ‘The vaccine virus must act in one or other of these two ways: either it must destroy the germe of the small-pox … or it must neutralize this germe’, Medical Journal 1803.
By the end of the century it was an accepted colloquialism for ‘harmful microorganism’. => germane, germinate
germ (n.)
mid-15c., "bud, sprout;" 1640s, "rudiment of a new organism in an existing one," from Middle French germe "germ (of egg); bud, seed, fruit; offering," from Latin germen (genitive germinis) "spring, offshoot; sprout, bud," which is of uncertain origin, perhaps from PIE root *gen-, *gene- "to beget, bear" (see german (adj.)). The older sense is preserved in wheat germ and germ of an idea; sense of "seed of a disease" first recorded 1796 in English; that of "harmful micro-organism" dates from 1871. Germ warfare recorded from 1920.