Psotník. Božec. Fraiß. Fraißen. These are the most common names for a death cause which is written in many records for dead children. And there were many children dying in the past, almost 50 percent of all children born. Terrible number, I know. But - what this death cause means? What is behind it?
Answer is - unfortunately - simple: absolutely anything that could cause a death of small child. Psotník (or Fraiß, if you are in parish book written in German) was simply any cause of death which couldn't be explained by epidemy (such as pox, measles, typhoid fever and more). Psotník is often translated as eclampsy or infantile convulsions, but it could have been anything else.
Psotník in Horní Záhoří death registry.
It could have been natural weakness of body. It could have been any congenial defect causing a death, such as heart or digestion failure. It could have been caused by a cold, high temperatures, infections, it could be any cause where the priest who saw the child was not able to tell what exactly caused the death.
Why? Because until the end of 19th century, only rarely was the children seen by a doctor. Only priests (or local reeve or mayor) decided what caused the death of anyone. And if the dead was child younger than 2 years, almost every time was the cause clear - psotník. It covered any possible cause of death. You'll most probably find information that psotník were convulsions connected to the lact of calcium or other minerals, but this is valid for the end of 19th century and later, not for records from the previous era.
I know I disappointed many people hoping they will find out what exactly siblings of their ancestors died of. Sorry for that - but psotník is just another of those generally used words, having thousands of meanings...