I remember having a textbook in jr high, possibly 10th grade, in the '90s, which had an SF section. One of the stories I have identified as “The Star Beast” by Nicholas Stuart Gray. The title of the other one I can recall eludes me.
It is about a small boy who is sick at home. As he gets sicker, he begins to imagine that the disease is taking him over and changing him, organ by organ, into something else. As each part is taken over, he ceases to be able to feel it. Eventually, it reaches his brain and everything goes black.
When he wakes up, he gets up and goes to touch his pet bird. He closes the cage and waits to see what will happen.
I can't remember what the other ones were. Some of the stories were illustrated with full-page blue-colour line illustrations.
Update: Identified another story from an answer here. "Zoo 2000".
A scan of the story from the textbook was included in a comment, which jibes with my memory.