They invade our bodies silently, hiding in the darkest recesses and draining our life from within. Once inside, they are in control. They live and breed inside us and can turn any organ in the body into their home. They eat us, change our DNA, rebuild our bodies to suit themselves and even alter our behaviour. It sounds like science fiction, but it is not. They are parasites, the most successful species on the planet. And we are their prey. Parasites kill one human every 10 seconds. Others keep us alive, for food and shelter.