The answer is algae.
Plankton includes a diverse group of organisms that live in the water, but are not capable of active swimming against the current. Therefore, since shark, squid and crab can swim and algae cannot, <span>among these groups </span>only algae can be a plankton.
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.