How does Iridescent Umbrella Badge work?

I don't really play Nemesis, so I got really confused today. 2 survivors get done healing and then one of them uses a vaccine basically in my face. I hit the vaccinated survivor and he only gets injured. What gives? There was another survivor near him, can you use vaccines on other people? If so, who gets exposed? The person using the vaccine (as per the description) or the person being cleansed?