Something that's systemic affects all parts of something. If every dog at doggy daycare has fleas, it's a systemic problem.
The adjective systemic is often used to describe diseases or disorders; a systemic illness affects your whole body or an entire system — like your digestive system. Any kind of system can experience systemic problems. For example, crime is a systemic problem in a community because it affects everyone from individuals to families, businesses, and tourism, just to name a few groups harmed by the problem.