Complex sentence: It's likely that a virus can pass through PFO therefore not being filtered by the lungs and sent directly to the brain, this can cause a type of sinusitus which can lead to viral meningitis.
Split into simpler sentences: It's likely that a virus can pass through PFO, therefore not being filtered by the lungs and sent directly to the brain. This can cause a type of sinusitus which can lead to viral meningitis.