Alien Strain — Michael Cole

An alien and his wife, whose distant ancestors were originally from Earth, decide to pay humanity a visit. Prior to their departure, they place a copy of their psyches inside a couple of spheres. The spheres are used on their planet to prolong life by having their neural network inserted into a clone. They hope that if something happens to them on Earth, and a human makes bodily contact with the spheres, they will be able to continue their lives inside a human host. When they land their spacecraft in Cambodia, a massive earthquake occurs.

Centuries later, his sphere is found. It ends up being sold to a wealthy businessman who wears it as an accoutrement to a masquerade party. Once the alien strain enters his body, the process goes awry, and a battle for survival begins. It's one thing for an alien to enter a clone and quite another for it to enter a living entity, particularly one that's human. The alien attempts to take over, but the human's immune system resists his advances. Eventually there can only be one victor. The question is, who will it be?