Some pathogens have little effect at all. Others produce toxins or inflammatory substances that trigger negative responses from the body. This variation means that some infections are mild and barely noticeable, while others can be severe and life threatening. Some pathogens are resistant to treatment..Viruses invade a host and attach themselves to a cell. As they enter the cell, they release their genetic material. This material forces the cell to replicate the virus, and the virus multiplies. When the cell dies, it releases new viruses, which infect new cells.