If you catch them at the wrong time the parasites are a race to be reckoned with. Every 200 years or so they travel the galaxy looking for new hosts, and we're not suggesting that this is a race that excels at making people feel welcome at parties. The hosts they are looking for will provide the much more vital role of keeping the parasites' immature larvae alive and prospering. For 198 years they have survived by feeding off a species whose own offspring have provided them with essential easily digested nutrients and regenerative salves. However having selected the most sumptuous specimens from the population, the host species has now been severely weakened and their nutrient content compromised.