The fact that the H7N9 virus has only recently infected humans helps explain why

Because the H7N9 avian virus acquired pathogenic mutations in order to infect mammalian hosts

Explanation:

H7N9 is a subtype of influenza avian viruses that has recently been reported to infect humans. Fortunately, H7N9 has also been reported to have a low pathogenic level; however, mutations might change this level to become more infectious for humans. Consequently, mutations in a given H7N9 strain might enable its adaptation and higher affinity to human-type receptors, being more aggressive to human cells, especially in lung cells, since this virus is characterized by rapid progression and respiratory failure.

answer:

i don't believe the ocean is purple, but it might once have been:

as for the sky, according to nasa:

"sunlight reaches earth's atmosphere and is scattered in all directions by all the gases and particles in the air. blue light is scattered more than the other colors because it travels as shorter, smaller waves. this is why we see a blue sky most of the time."