Great question! As I understand it, 'infectious' and 'affectionate' derived from Latin verbs with different endings, hence the inconsistencies. 'Inficere' (to taint, corrupt) gives us the basis for 'infect', while 'afficere' (to affect or influence) leads to 'affective' or 'affectionate'. Latin language rules are complex, but this is the gist! …