Cold Viruses are small, microscopic monsters that can infect people and make them sick. King Dedede wanted to become sick just like everyone else, so he bought a jar full of these tinsy winsy little creatures and swallowed them up. Noticing that he could finally sneeze and stuff, Dedede was overjoyed at first, but soon realized that being sick was not fun at all. So he picked up Kirby and bought a Microtizer from Nightmare Enterprises, which he shrunk Kirby with so he could send him down into his stomach to fight the monster germs. Before Kirby could inhale all of the Cold Viruses, Dedede sneezed once more, and therefore he was left to end up having to put up with being sick for a while longer.
Physical Appearance
Cold Viruses aren't very hard to describe. They are round purple spheres with flat red-orange eyes and two groups of three smaller sticks sticking out from both sides. Cold Viruses are microscopic, meaning they are too small to be seen with the naked eye. Since they are that small, they tend to be found in incredibly large groups.
Abilities
Basically, just like their names, Cold Viruses are very germ-like and can infect people and make them sick. At one time, King Dedede also imagines one Cold Virus growing to about Dedede's size and then electocuting him, causing him to catch its cold, and the virus supposedly looks as though it is winking both eyes, though it has no eyelids. However, the NME Salesman corrects that by saying to him that he'd have to drink the whole jarful of the lots of Cold Viruses to get sick. Although it's been said that only smart people get sick, viruses, especially the Cold Viruses, don't care wether anyone is smart or not, and just infect them anyway.