The infected humans usually shape-shifted back to their original shape and kept acting like they used to act. Very little changed, EXCEPT that they now strongly believed that everybody should be infected by the aliens. This was very different from how most people felt!
Infected humans could infect other people, but only if the other human was willing to be infected. But the infected humans saw no problem with lying like crazy or manipulating people to get them to "agree" to be infected. My infected pharmacist wouldn't fill my prescription for antidepressants. She said that if I got really depressed, I would long for the community provided by being infected by the aliens.