Opposing or preventing the action or development of pathogens; having the quality of fighting disease-causing organisms.
From anti- + pathogenic (disease-producing). The -ic suffix converts this into an adjective form, which became the standard medical term rather than the noun antipathogen, following medical English's preference for -ic adjectives.
Antipathogenic is the 'official' form you'll find in modern medical literature, while antipathogen (the noun) feels slightly old-fashioned. This shows how words compete for dominance—the adjective form won this particular battle.
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