Some infections will be virulent even for people even with optimally functioning immune systems, if it's the kind of infection that acts faster than anyone's im

Some infections will be virulent even for people even with optimally functioning immune systems, if it's the kind of infection that acts faster than anyone's immune system's ability to respond, or if it's a new strain that no human's immune system can defend against. But that aside, the virulence of an infection will always be greater against those who have weaker immune systems. This is why we always see the elderly and those with pre-existing serious illnesses more likely to be struck down or killed by infections. Stress is just another factor that can lead to impaired immunity and result in an infection becoming virulent.