Full of disease; causing or spreading illness; sickly and unhealthy.
From disease + -ful (suffix meaning full of). This old English formation follows the pattern of other -ful adjectives like 'harmful' or 'joyful', combining a noun with the fullness suffix.
Doctors in the 1600s described plague-infested towns as 'diseaseful' to warn people away—the word itself was meant to sound unpleasant and repugnant, making you want to avoid the place.
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