Why did the author didn't get the housemaid's knee(arthritis) ?

We aren't told exactly why he hadn't contracted housemaid's knee, but he admits that after wondering why...... he was determined to do without it. Because his illnesses were all in his head.... this was simply an illness he decided to bypass.

I plodded conscientiously through the twenty-six letters, and the only malady I could conclude I had not got was housemaid’s knee.

I felt rather hurt about this at first; it seemed somehow to be a sort of slight. Why hadn’t I got housemaid’s knee? Why this invidious reservation? After a while, however, less grasping feelings prevailed. I reflected that I had every other known malady in the pharmacology, and I grew less selfish, and determined to do without housemaid’s knee.