The film, itself, was based loosely upon a real-life incident involving Cheeseman Park in Denver, Colorado. The town built the park on a cemetary, but ran out of money to move the bodies. So they just left a few corpses there, underneath the sod, presumably next to a sign that read "no digging in the sandbox."
The main piece of Poltergeist lore is that the prop guys decided to use real cadavers, but not tell the crew or cast. It's on this web page, which looks like it was made in 1995, so it must be true. Apparently real skeletons are cheaper to produce than plastic ones. We're confused as to why these prop guys were suddenly so thrift-conscious: The Spielberg production had a ten million dollar budget, which was no small potatoes, at the time.
So, you're using real corpses in a film about violating an Indian burial ground. Something has to go wrong, right? The first sign that there might be a curse was when actress JoBeth Williams would return home every day, after filming, to find all her pictures were askew. Apparently, this is a subtle warning from spirits that people are about to start dying.
In the six years between the first and third film's release, four of the principal cast members died, But those deaths are just one of several instances that made the production team fear a curse. After the first days' shooting, the film came backentirely blank. There was also an abundance of minor injuries due to actors and crew members tripping on set. The writer of the novelization of the book had his apartment struck by lightning, blowing up his air conditioner. That's exactly what happens in a dead corpse curse people; look it up.