Daveigh Chase's died from complications due to AIDS with chronic polysubstance use as a contributing factor, according to the Los Angeles Medical Examiner's office.
https://www.foxnews.com/entertainment/lilo-stitch-actress-daveigh-chases-official-cause-death-released-two-weeks-died-35
https://www.foxnews.com/entertainment/lilo-stitch-actress-daveigh-chases-official-cause-death-released-two-weeks-died-35