Because they've sucked dry their hosts in the West, and globalism allows them to move on to another region. With "vertical transmission", a parasite must stick

Because they've sucked dry their hosts in the West, and globalism allows them to move on to another region. With "vertical transmission", a parasite must stick around to deal with the consequences of its actions to the host, so such arrangements evolve into benign symbiotic relationships. With "horizontal transmission", a parasite is free to kill its host and move on to another. Such parasites are noted for their virulence.