Disease

A disease (病気, byōki) is a condition in which a human or animal causes an abnormality in the body, resulting in some kind of malfunction.

As a pixiv tag, it means the following:

• A picture that the author is a little bit sick in the head.
• A picture that the author drew when they're really in a state of illness such as a cold.
• The person in the illustration is ill.

In the case of tag 1, this tag is used less frequently because there is a tag, author is sick, which has the same meaning.

However, there is no need to make a clear distinction between the two tags, so you can use whichever you prefer.

Sick　Sickness　Ill　Illness

病気