The Science of parasitology is a fascinating subject that should be explored here to understand "why". Our bodies and their processes have been messed up for "hundreds of years". Actually, what's called Miasms in homeopathy is relevant here as pathogenic RNAs are passed to the foetus, which inherits Miasmatic predispositions, i.e., underlying diseases. So, flu RNAs, for instance, from different historical types that our ancestors had are in our bodies from the beginning of our lives. They are manifested in different ways later on, i.e., symptoms.
The notion "No symptoms = no disease" is wrong at its roots.