Why do you think parasitism is such a runaway success? What are drawbacks of being a parasite?

Well, for a drawback, you obviously do need a host, and so you are fairly reliant upon the availability of host organisms. The reason for their success is probably due to the fact that being a parasite allows you to hijack certain elements of the host’s internal mechanisms, and use up various products without doing the hard work to convert them in the first place. This allows parasites to be relatively simple and not expend lots of energy on things like metabolism.

I suppose another benefit is that once you’ve found your host, your pretty much set; no further energy expenditure necessary till your host dies or you are forcefully ejected. Also, the insides of other animals is a very good place to raise little parasites; plentiful warmth and food!

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