Natural selection doesn't favor traits that are somehow inherently superior. Instead, it favors traits that are beneficial (that is, help an organism survive and reproduce more effectively than its peers) in a specific environment. Traits that are helpful in one environment might actually be harmful in another.
(one again, I hope this helps ^^)
Pathogens generally invade the body, in their invasion they can duplicate and cause confusion such as hijacking your cells, thus they cause infection and disease. Some of these pathogens are spread through air, sweat, blood, excrete (urine/feces), or general human contact with anything concealing the pathogen.
The main organs of the Central Nervous System are the Brain and the Spinal cord/Column
