'infectiousness' originates from English, specifically formed by adding the suffix '-ness' (meaning 'state or quality') to the adjective 'infectious', which itself comes from Latin 'infectiosus'.

'infectious' changed from Medieval Latin 'infectiosus' (from the past participle 'infectus' of Latin 'inficere') into Old French forms such as 'infectieux' and then into Middle English as 'infectious'; the modern English noun 'infectiousness' was formed later by adding '-ness' to 'infectious'.

Initially it carried the sense 'causing contamination or corruption' (from Latin usage), and over time it came to mean specifically 'capable of being transmitted (causing infection)' and was extended figuratively to mean 'easily spread' (as with feelings or ideas).