Why milburn did not get pregnant by oral hammerpede attack?

Holloway was infected by David with one drop of the black goo. His sperm seems to be mutated so shaw became pregnant with the squid. So far so good... ------------------------- The worms mutated because they layed in the black goo to the cobraalien/hammerpede. Then one of them attacked milburn and killed him. Was hi pregnant with chestburster or something or he was not? And why the other snake did not attack Fifield? No interest, no hunger? --------------------------------- Strange that there where more then one cobra aliens - so the worms had the same mutation from the black goo to cobraaliens, so the mutation is not by random (that one worm becomes cobraalien, and the other worm mutates to something else..)... So humans infected by black goo would all turn to some zombies with physical strength and animal agressivity, if the engineers would through the black-goo-containers onto earth.