The Blue Virus from the Dr. Mario games makes an appearance in Mario & Luigi Bowser's Inside Story.
A Blue Virus is of course the blue one in the picture above. (Thanks to Mariowiki for pic)
The first place you can find it is in the building where Dr.Toadley is in, located in Toad Town. It's called the Toadley Clinic. Inside the building, a poster above some books shows a "Blue Virus" in a circle with a line through it...of course they don't want sickness to spread.
The second place you find them is on Plack Beach. Go near a Toothy, but don't get into a battle with it. You'll see that it will open its mouth a little. Then, it will spit a "Blue Virus" out at you so you will get into a battle with it. That's not good for your teeth...