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In the Pokémon world, several Pokémon have been shown to develop illnesses, similar to what happens with the status conditions.
In the games
Prior to the events of Pokémon Black and White, Alder's starter Pokémon died of an illness, and in despair over not being able to save it, he began traveling around aimlessly. Before his partner's death, the two of them madly pursued strength, much like Cheren did. The Pokémon's death changed Alder's outlook on life. The identity of this Pokémon is never mentioned in Black and White, but dialogue in Pokémon Black and White 2 reveals that Alder's late partner was Larvesta/Volcarona.
In the anime
Pikachu's illness of electric overcharge
This condition is known to affect Electric-type Pokémon, although Magnemite and its evolutions appear to be immune to it. This illness is caused by exposure to high levels of electromagnetic energy. Symptoms appear to vary, as shown by different species stricken with it. In Hoenn Alone!, Pikachu came down with this illness. It appeared quite lethargic when affected, whereas a wild Elekid from Reversing the Charges did not. It also appears that there are different levels of this illness, as the first time Pikachu got it, it only had flu-like symptoms; a different time, however, it got so severe there was a threat of Pikachu exploding from the overload. A Pikachu that has an electric overcharge usually becomes red-eyed, aggressive, and dangerous toward its Trainer, though this may only apply to severe cases.
Dawn's Pachirisu got an electric overcharge in Doc Brock!, where it was able to get rid of it with an Oran Berry and attacking Team Rocket. A wild Eelektross also got affected from overcharge after becoming lost and frustrated in the subways of Lumiose City in SS037. It was calmed down by Dedenne's Nuzzle which helped it discharge its electricity.
This condition is cured by discharging electricity, usually via moves, until the Pokémon's electricity level is normal.
Known to affect Pokémon of different types, fevers have appeared frequently in the anime. Meowth developed a type of fever in The Problem with Paras. His was cured fairly quickly by a mixture of herbs, as well as pills taken with meals afterward.
A more severe fever struck a wild Ralts in Do I Hear a Ralts?. It was treated by Ash and his friends giving it food and keeping it cool until arriving at the local Pokémon Center for more intensive treatment. The nature of the treatment was unclear, but it appeared to include the use of an oxygen mask. The recovery was quick, and no follow-up treatment was necessary.
Both Meowth and Ralts became listless when affected, although it was more variable in Ralts.
James's Chimecho also had a large fever in Sweet Baby James, in which James had to leave Chimecho at his grandparents' house near Saffron City. He plans to return there to get Chimecho in the future.
Known to affect humans in the Pokémon world, this malady causes them to think and act like various Pokémon. The only known cause of Pokémonitis is being exposed to a Hypno's hypnotic waves, and even then, only when the Hypno has been trained so that its Hypnosis attack specifically affects humans. It is curable by Drowzee's Dream Eater, as seen in Hypno's Naptime. Adults appear to be immune to Pokémonitis, as the oldest known person to be afflicted with it was Misty, despite numerous adults being in extremely close proximity of said hypnotic waves.