Scabies vs. Measles: What Is Unseen is Worse Than What is Seen

It's often said that when you see healthcare workers practicing meticulous infection control the diagnosis of the patient being cared for is one of two things: lice or scabies. For some reason, ectoparasites (macro) visible to the naked eye strike fear in the hearts of the world. While an invisible potentially lethal virus like, for instance, measles is something to have a party for and wish on partygoers. 

Another ectoparasite that also merits this misplaced terror are bed bugs, which are really are incapable of transmitting disease to humans (with the possible exception of Chagas Disease in certain contexts). Case in point: a local community college in the Pittsburgh area cancelled classes  because a student had bed bugs in his home! 


The action was taken "out of an abundance of caution" -- the familiar phrase used to justify serious threat misperceptions and excuse the performance of response actions for which there is no evidence.

Similarly, a recent episode of a television program mentioned scabies as a "complication" of promiscuity when much more dangers pathogens such as syphilis, gonorrhea, and chlamydia (not to mention HIV, hepatitis B, and HPV) shows how widespread this phenomenon of prioritizing macro ectoparasites reaches.

What accounts for this threat misperception?

My own speculation is that it is the same thing that stalled the development of the germ theory until more powerful microscopes were developed: people are much more accepting of what they can see vs. what they have to imagine, infer, or abstract. In other words, the more concrete an entity (like a scabies mite) seems the more it can be evaluated and judged; conversely, the more abstract an entity is (like a virus) the more distant and unreal it seems. This cognitive bias can be harmful because what isn't seen, in the case of infectious diseases, can be deadly.