NatJones

When sick, your immune system struggles to fight against an infection.

A jab works similarly to this: they insert components of bacteria/virus, etc. inside your organism to make your immune system react to it and produce antibodies. So, they actually infect you with something, but it is so weak that the only thing it can do is trigger your immune system (aka make you experience very light symptoms, e.g slightly increased body temperature).

I you have a weakened immune system due to it already fighting against an infection, infecting it even more isn't optimal, even with the extremely weak infectious components that are included into the jab.