Complex question! The past participle stem of 'inficere' (to dye or poison) is 'infect-', thus 'infectious'. On the other hand, 'affectionate' comes from 'affectio', the noun form of 'afficere' (to have influence on). The '-ate' ending is common in forming adjectives off of verbs with '-io' endings. 'Affectious' would arguably …