What causes a weakened immune system?

You can be born with a genetically altered/weakened system. You can acquire a variety of infections/conditions that weaken it over time. You can be exposed to drugs by chance or from treatments that weaken it (often used for transplant patients).

There are far to many circumstances that can result in immune deficiency. Visit this site for info: https://www.webmd.com/a-to-z-guides/immune-deficiency-disorders#:~:text=This%20low%20activity%20is%20called,combined%20immune%20deficiency%20(SCID).