Disease
Sickness, Illness
noun, suffix
As you can probably imagine, when you add the kanji sick to another word, you're saying what kind of sickness it is; you are mentioning what disease it is.
Since this is attached to other words, it's treated like jukugo and gets on'yomi treatment. This is the reading you learned with the kanji.
重病
serious illness
大病
serious illness
うつ病
depression
アルツハイマー病のせいで、色々な事を思い出せない時があります。
There are times when I can't remember various things because of Alzheimer's disease.
重いメニエール病になってから、もうすぐ三年です。
It's been three years since I got severe Meniere's disease.
うつ病と上手く生きていくためにも、心体が元気でいたいです。
In order to deal with my depression, I want to stay healthy physically and mentally.