Disease, Ailment, Disorder
noun, の adjective
Something that you are afflicted with rapidly is a disease. You should go to the doctor's right away to treat this fast-acting ailment.
しっかん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
私の妹は、精神疾患を抱えています。
My little sister has a mental disorder.