Cold Viruses are small, microscopic monsters appearing in the episode Something to Sneeze At in the anime Kirby: Right Back at Ya! that can infect people and make them very, very sick. King Dedede wanted to become sick just like everyone else, so he bought a jar full of these monsters from Nightmare Enterprises and swallowed them up. Noticing that he could finally sneeze and be ill, 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, with 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 Kirby out, and he ended up having to put up with being sick for a while longer.
Cold Viruses are round, purple spheres with flat, red-orange eyes with two groups of three small sticks sticking out from both sides. Cold Viruses are microscopic, and they tend to be found in incredibly large groups.
Cold Viruses are extremely virulent and can infect people and make them sick. At one time, King Dedede imagines a Cold Virus being a monster of about his own size, electrocuting him and winking its lidless eyes at the same time, causing him to catch cold. However, the NME Salesman corrects him, and says that he'd have to drink a whole jarful of Cold Viruses to get sick. He proceeds to say that although it is said that only smart people get sick, viruses, especially the Cold Viruses, are indiscriminate, and just infect them anyway. The NME Salesman could have said this purely as a veiled insult to Dedede, implying that he isn't very smart.