Parvo is a virus and worms are parasites. With some parasites it can be difficult to tell the difference from parvo. Some are obvious worms and visible to the naked eye. Others have similar symptoms to parvo and can only be seen with a microscope. Testing is run for Parvo. Parasites and Parvo and even infections and bacterias can all have similar symptoms but some can be treated a little more easily than others if it's caught soon enough. Vomiting, Diarrhea (often with blood and even mucus), depression/lethargy, lack of appetite and not drinking much/dehydration are all signs of many illnesses (and if they ate something they shouldn't and it's causing a blockage in the intestines though they can also stop pooing if that's the case) a puppy is prone to including parvo and parasites so if your concerned about a puppy I'd go to the vet because we can't run those tests at home and the treatments are different so you can't take care of it until you know what it is.