I legitimately believe it is wholly possible semi-advanced civilizations have come and gone from Earth and we've yet to find trace of them.
Without modern medicine an infectious disease could cripple a civilization to the point of becoming barbaric again, look at the Great Plague... 30-60% reduction in European populations in FOUR years, now imagine something similar happening in a moist tropical environment. Or even look at the Corrupted Blood Incident https://en.wikipedia.org/wiki/Corrupted_Blood_incident which shows (in a game though) how quickly infection could spread if you have a means of rapid travel. A civilization that lacked motorization but was an advanced sea-fairing society, again in a tropical region, could spread a disease at frightening pace.
When we find evidence like this it just reinforces my opinion that past civilizations were incredibly possible, even at a level advancement of up to the 16th or 17th century.
Look at the Islamic Golden Age, they were basically scienctists and explorers that worked on collecting all the world's knowledge and even churning out their own discoveries in philosophy, mathematics, natural sciences, medicine, engineering, then BAM! They went from this incredibly knowledgeable society to simple shepherds and the like within a century in places. Imagine if you tossed in a highly contagious outbreak in there...
Neat stuff.
