parasites are those that prevents others from succeeding so that they can succeed themselves

those humans that go on welfare or don't study hard or are lazy at work are not parasites in a population 

humans that are parasites in a population are those humans that: 

friends that prevents a friend from dating another person so that they can succeed themselves in dating 

collegues that prevents another collegue from succeding at work so that they can succeed themselves 

parasitism and competition are different things