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Why can a parasite that becomes too successful harm its own population?

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Müllerian mimicry: butterflies — mimicry *Heliconius* butterflies demonstrating Müllerian mimicry, a form of mimicry where one or more species exhibit closely similar warning systems. In this case, wing patterning and coloration among the species appear very similar.

A parasite that becomes too successful—specifically by evolving high virulence—can inadvertently harm its own population by killing its host too quickly.12

For a parasite to survive and propagate, it must successfully transmit from one host to another. If a parasite is so virulent that it causes the swift demise of its host before it has the opportunity to spread, the parasite population effectively dies along with the host. This outcome is of negative survival value to the parasite, as it eliminates the very resource—the host—required for its continued existence.132

Natural selection generally favors a balance between virulence and transmission. When host population density is low, parasites that kill their hosts too rapidly are often selected against, as they fail to find new hosts, leaving only less virulent forms to survive. Conversely, if a host population is dense and transmission is easy, higher virulence may be favored because the parasite can multiply quickly and escape to new hosts before the current one dies.1