Not for diseases like parkinsons. Parkinsons is a degenerative disease or wasting disease where brain cells dye and are not replaced properly. We still don’t really know the cause of parkinsons is yet but we know you can’t catch it from other people so its unlikely you could inject it into someone.
You could however inject bacteria or viruses into someone to make them sick.
That would be pretty nasty though…. please don’t inject people with diseases!!!