EM Viruses (電波ウィルス Denpa Wirusu) are viruses in the Mega Man Star Force series that plague the Wave Road and the Earth since it is covered with EM Waves. They cause disturbances in the Wave Road which cause machines and anything running on EM Waves to break down.
Viruses are enemies commonly found through random encounters whilst on the Wave Roads. These viruses often appear in groups of two to three, and sport low HP and an inability to flinch or enter invisibility like bosses. Deleting virus groups will net the player a reward, usually Zenny or a Battle Card given from one of the viruses, depending on the Virus Busting rank.
The player can also encounter "G" versions of these viruses, which sport double HP and double damage (in Mega Man Star Force 3 the multiplier is x1.5 instead). These viruses are encountered in the first Mega Man Star Force game in plot-relevant encounters and do not yield any reward upon defeat; in Mega Man Star Force 2 they will yield Star Cards that power up Battle Cards of the same name; in the third game they appear in Warning Battles, give only Zenny, and allow Mega Man to stand a chance at gaining a Noise Change.
Viruses can also be located and battled in the real world whilst as Mega Man, in the third game. Heading up to them and pressing the A button will launch Mega Man into a fight, with a reward at the end as usual depending on the Busting rank. Defeating all viruses in the real world is a requirement to opening one of the doors in the secret area of said game.
In the first game, at the very start of the game, an old man in Echo Ridge will tell Geo Stelar that the Satellite Admins are responsible for deleting viruses before they land on Earth, implying that EM Viruses originated from Outer Space, and there were none on Earth originally. Also, Battle Cards were created three years prior to the first game in order to delete the EM Viruses who had been showing up. Jammers are human infected by EM Viruses, which means they can fuse themselves with humans, just like FM-ians can EM Wave Change. It is implied the EM Viruses in the first game are most likely FM-ians themselves.
In the second game, however, there are new viruses that appear due to Mu's rise, which means there were EM Viruses on Earth, although most of them were apparently in "suspended animation" prior to Mu's second rise.