Illness in the Pokémon world
In the Pokémon world, several Pokémon have been shown to develop illnesses, similar to what happens with status conditions. Pokérus is an unusual example of a disease with effects that are beneficial. Additionally, some people have also been known to get sick or suffer from illness.
Pokérus
Pokérus is a rare viral infection that causes Pokémon to grow exceptionally well. Specifically, it doubles the effort values a Pokémon gains from battling. The infected Pokémon can infect other Pokémon with the virus for a period of one to four days before becoming cured. While similar in many ways, the Pokérus is not a status condition and cannot be healed at a Pokémon Center or with any status condition healing item. Eggs may also catch the Pokérus like any other Pokémon. Once a Pokémon's immune system has fought off the virus, they cannot spread it further, nor can it be spread to them by other Pokémon. However, the Pokémon still gains double effort values when cured.
In the anime episode Oaknapped!, Dr. Namba explained to Professor Oak that the mysterious viral lifeform Pokérus is an important component of his Pokémon Power Acceleration Project (PPAP), a secret project that aims to infect Pokémon with the Pokérus to evolve them at a faster rate. However, all of his specimens had died out. Professor Oak also explains that the term "Pokérus" was coined by combining the words "Pokémon" and "virus" together. Later, Professor Oak began researching about the Pokérus and Team Rocket's true intentions of using it.
In the Platinum arc of the Pokémon Adventures manga, the Pokérus was discussed in The Final Dimensional Duel IX, when Platinum's team was discovered to have this virus and, as a result, became stronger. Her Froslass, Pachirisu, and Cherrim had it prior to being taken from the hospital, and then it spread to the rest of her party.
In the games
In the Generation II games and their remakes, Jasmine's Ampharos, Amphy, is found ill in the Glitter Lighthouse in Olivine City when the player first arrives. At the top of the lighthouse, Jasmine asks the player to retrieve the SecretPotion from Cianwood City to cure Amphy.
In the Generation III games and their remakes, Wally is a sickly young boy who lives in Petalburg City. Despite his illness, his dream is to train Pokémon. His illness is never explained, but his symptoms appear to clear up in clean air, suggesting that it is either acute bronchitis or asthma. He first appears in Petalburg City, asking Norman, the Petalburg Gym Leader, to help him catch a Pokémon before he moves to Verdanturf Town to live with his aunt and uncle. This is because Verdanturf is in a remarkably clean area and might improve Wally's sickness. Wally eventually runs away from home and challenges the player at Victory Road, seemingly recovered from his illness.
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. 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 Black 2 and White 2 reveals that Alder's late partner was a Volcarona, which evolved from a Larvesta.
Pokédex entries
According to its Pokédex entry in Pokémon Sun, if the swirl on Poliwag's belly is tinged white, that means it's affected by some disease.
In its Disguised Form's Pokédex entry in Pokémon Shield, it is said that a scientist who peeked under Mimikyu's old rag in the name of research later died of a mysterious disease.
In the anime
Main series
Electrical overcharging
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 after Team Rocket used an electromagnet in an attempt to drain Pikachu's electricity. It appeared quite lethargic when affected, whereas a wild Elekid from Reversing the Charges did not. He was fully healed in the following episode by a giant mecha that the trio used to try and drain Pikachu's electricity, but only drained the excess energy out of him rather than what they intended. This event shows 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 XYS06. 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.
Fever
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 they arrived 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.
Pokémonitis
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 person to be shown afflicted with it was Misty, despite numerous adults being in extremely close proximity of said hypnotic waves.
Other illnesses
Chespie's coma
In Mega Evolution Special IV, Chespie entered into some form of coma after being exposed to the energy that a Zygarde Core (Squishy) had been subjected to, devastating Mairin. Much later, in The Synchronicity Test!, Chespie had still not recovered, but Lysandre claimed that Alain could revive Chespie by collecting Mega Evolution energy. In truth, Lysandre had no intention of helping the Spiny Nut Pokémon; the energy was actually to power a machine to influence the mind of Zygarde. After Lysandre's true colors were exposed, Mairin set out to rescue Chespie from Lysandre Labs, with the help of Serena and Professor Sycamore. Chespie was eventually rescued in The Right Hero for the Right Job!. However, it ended up absorbed by Lysandre's fallback plan, a Giant Rock with the appearance of Zygarde 50% Forme, in Rocking Kalos Defenses!, although it was eventually rescued by Ash and Alain in Forming a More Perfect Union!, temporarily halting the Giant Rock. The energy was absorbed by the Giant Rock, and as a result, Chespie finally recovered.
Meowth's cold
In Pikachu Translation Check..., Meowth, wearing a medical face mask, revealed to Jessie and James that he had a cold. Jessie and James suggested he get some rest, offering him warm blankets and a cool towel. Jessie asked Meowth if he wanted anything to eat, but Meowth suspected his teammates planned to slack off while he was sick as they listed a heap of treats for themselves. In his weary state, Meowth declared that there was one thing Team Rocket was missing, but he collapsed before he could reveal the answer. Later, Jessie tried to shake Meowth awake, hoping Meowth could review her latest scheme, but Meowth instead fainted from illness again. Meowth eventually recovered after a day of rest.
Unspecified illnesses
Before To Find a Fairy Flower!, Florence had one day suddenly fell ill and collapsed while she was gardening. She was quickly taken to the Calanthe Town Central Hospital by ambulance. This saddened her husband's Flabébé, who had always enjoyed Florence's company, and she left to search for her. While Flabébé was looking through the window of the hospital Florence was staying in, a strong gust blew her away, and her flower was run over by a passing car and destroyed. Grey was unable to retrieve her, and as such, Florence went into a state of emotional despair despite her treatment being successful.
At some point prior to A Watershed Moment!, Henny's grandfather, Chapman, died of a certain illness.
Prior to the events of Pokémon the Series: Sun & Moon, Mallow's mother passed away due to an unspecified illness while Mallow was still little.
In The Power of Us, Kellie was a young girl who suffered from an unidentified illness that affected her energy levels and could be triggered by stress.
In JNS05, Ash met the ghost of a boy named Haruto who had died of an illness before getting to set off on his journey as a Pokémon Trainer.
Pokémon: Twilight Wings
John was a young boy who was being treated for an unknown illness in a hospital, where he met his good friend Tommy while they shared a wardroom. John hoped to become a great Pokémon Trainer just like his idol Leon once he recovered from his illness.
In the manga
Pokémon Adventures
In Pokémon Adventures, Giovanni had long suffered from an unspecified debilitating disease before being cured with medicine made from herbs gathered by the Team Rocket Elite Trio with the help of Gold's Pichu, the Spiky-eared Pichu, and Celebi.
HeartGold & SoulSilver arc
In All About Arceus II, Lance told Bugsy the history of the Pokéathlon's creation. The tale consisted of an island plagued by illness and a Pokémon Trainer setting off to find a Mythical Pokémon that could give them the cure. Once the Trainer and their Pokémon became one, only then did Celebi, the Mythical Pokémon they had been searching for, give them the cure. The Pokéathlon was created to celebrate this Trainer's achievement; the sports are meant to represent the obstacles the Trainer had to go through in order to find the cure.
Pokémon Gold & Silver: The Golden Boys
In A Promise Given to Miltank, after Whitney's Miltank had been diagnosed with a deadly disease that has been spreading to various Miltank in the region, Gold, Whitney, and Morty set out to find the Berries required to cure her, only having time until sunset before the disease would claim Miltank's life. Their Berry-gathering was hampered by wild Pokémon appearing from the Berry trees, but ultimately, they managed to find a tree with lots of Berries on it. Gold used his Sudowoodo to lure away the Heracross that were swarming around the tree, while Whitney had Pikachu knock the Berries down with Headbutt. The Berries were delivered to the Ecruteak City Pokémon Center just in time, saving Miltank's life. Out of gratefulness, Whitney gave Gold a Plain Badge and returned his Pikachu to him.
Pokémon RéBURST
In Pokémon RéBURST, Karuta's home village was plagued by a disease that had claimed several of his friends and family. Karuta entered the Burst Warrior tournament, Burst Heart Survival to win the prize money and help the sick people of his home village. The final match ultimately ended in a tie between Ryouga and the Boss of Great Gavel, Fraud. Great Gavel left the area and left the prize money there. After returning all of the stolen Burst Hearts, Ryouga, Miruto, and Yappy traveled with Karuta to buy the medicine he needed to heal his dying village. After buying the medicine, Karuta and the others were attacked by a Great Gavel member named Rūkamu.
After Rūkamu sent Miruto off of the bridge, Ryouga ran to go rescue her, leaving Karuta alone to face the Scraggy Burst Warrior. Despite his best efforts, Karuta was unable to land any hits on Rūkamu and was angered when Rūkamu called him "Crybaby Karuta", a nickname that he was referred to in his village. Eventually, Ryouga returned with Miruto and defeated Rūkamu by sending him flying into the mountains. After defeating Rūkamu, the group proceeded to the village but, upon arrival, Karuta told them that he didn't want to go into the village.
After explaining to them how he got his nickname and who his father was, Ryouga tossed Karuta into the village and gave him words of encouragement. Moved by his words, Karuta gave the medicine to his dying village and healed the sick people. Unfortunately, when Karuta arrived at his house to see his mother, he found that she had died that morning.
In other media
Detective Pikachu
In Detective Pikachu, Howard Clifford was diagnosed with a degenerative disease that would soon leave him wheelchair-bound. Upon receiving this diagnosis, he resigned his position within the company and passed it to his son Roger, then set out on a journey of the world in search of a cure for his condition. His research led him to the conclusion that if humans could evolve into the best versions of themselves the same way Pokémon could, it would solve such issues.