Two senses by context: medical 流行性感冒 (influenza, flu), 流行性腮腺炎 (mumps), 流行性脑膜炎 (epidemic meningitis); cultural 流行性 (popularity, mass appeal). The shared root is 流行 (liúxíng) = 'to circulate / to spread', plus -性 turning it into a property.
memory
Decompose: 流 (flow) + 行 (move) + 性 (-ness) → 'flow-and-move-ness' - the property of spreading widely, whether of viruses or pop songs.
