The answer is Sensory Receptors
Earth and its atmosphere are continuously altered. Plate tectonics shift the continents, raise mountains and move the ocean floor while processes not fully understood alter the climate. Such constant change has characterized Earth since its beginning some 4 billion years ago.
Viruses can multiply very quickly, and so they can go through natural selection at a much quicker rate, allowing certain vaccine resistant mutations to take over a population.
Number one is sex cells are created