The Hiss manifests in human hosts in numerous ways. We see the most common and least intense form in the many corrupted agents floating throughout the Bureau.
The Hiss Agents have maintained their human appearance, undergoing no obvious physical transformation. The most notable distinction from an unaffected human is the fact they levitate and are completely unresponsive.
These Hiss have displayed no aggressive behavior as they only seem to vocalize the strange incantation ad infinitum. These vessels are possibly only meant to propagate the Hiss corruption, like spores. They could also be cocoons, preparing to enter the next stage of their evolution.
When attacked, Hiss Agents disappear. It is unknown whether they enter a state of invisibility or undergoing a transformation into a purely resonant form.