Brief Response Pokemon, according to the anime, may live for years in a Pokeball while being completely awake. Ash, on the other hand, was once worried that his Pokemon might freeze within their pokeballs. However, all of his Pokemon survived being put in their balls during winter.
At the very least, the wild ones perish when their health is depleted. The Pokemon are tethered to that one pokeball, which monitors their vital signs and pulls them back in when they are about to die ("fainted"), so they may be cured and put into the next deathmatch.
However, since there are only eight slots available in the battle box and many more Pokémon than that out in the field, it's possible some don't get saved. Also, since the matchmaker picks who goes into what battle, not all fainted Pokémon get matched up.
Finally, since some Pokémon have special abilities such as "noise" or "darkness", they can affect how others rate them. For example, if a Pokémon with noise runs out in the field, it may not receive a call from the matchmaker, even though it's fainted.
These are just some of the ways in which lost battles occur; there are many more factors to take into account. But these should give you an idea of how accidents happen in the game.
Pokemon do not technically die during gaming. Nothing will "kill" your Pokemon in any game, no matter how many times they are knocked out, how much stronger the other Pokemon are, or how powerful the attack performed was. Pokemon do die in the Pokemon universe, in some form or another.
When you trade or transfer a Pokemon from one game system to another, it loses all of its stats and abilities in the process - including its health. So if you want to keep a Pokemon alive while trading it, you have to give it some kind of gift first - such as a Pokemon Stone or a Game Boy Camera. Then when you get it back, you can use the Gift Item function on your GBA System Menu to make the Pokemon receive full health.
However, even with these items, your Pokemon will still be at risk of death in these trades. If there is even a chance that something could happen to it during the trade, for example if you send it through the mail, then it has died. This means that even if you give your Pokemon every possible advantage in terms of health, they can still be killed by something outside of your control.
In order to truly kill a Pokemon, you would need to destroy all of its parts - including its brain. But since this isn't possible, we can only assume that if a Pokemon is unable to move or speak, then it has died.
Apparently, the pikachu did not enjoy remaining inside the pokeball, and it did not like Ash at first, so it disobeyed him. (This is a synopsis of EP001 "I Choose You!") Also, in the anime episode Pikachu's Rock Tomb, when Ash's team was attacked by Rotom, who used its radio to contact other rotoms everywhere, one of them went to Pikachu's ball and opened it, causing Pikachu to attack it.
In the manga, this story happens after chapter 5. In that case, Pikachu just hates being locked up, but instead of attacking the pokeball, it uses its electric attacks on the walls of the room where it is trapped.
Also, in the same manga chapter, we can see that Ash's pikachu has different colors for its tails, which means it wasn't born with those colors. This proves that the pikachu knows it's own name and will use it if it feels threatened.
This isn't necessarily true for the rest of the Pokemon in the world, as a handful have died and gone to the afterlife. Pokemon who have died have appeared in video games, comic books, and anime. From the Stoutland, who died of old age, to the Marowak, who was killed in battle against Team Rocket, many Pokemon have left this life behind. However, most deaths in the anime are caused by other people, such as when Ash's Pikachu is killed by a Sawk in The Iron Ball Challenge.
Some characters in the anime have also died. For example, a young Serena's Umbreon died after being hit by a car. Another death that got a lot of attention from fans was the murder of Juniper's Piplup by a trainer in Black & White. This does not mean that all characters in the anime are safe - far from it! Many characters have been injured or taken sick. In addition, some locations have been destroyed by disasters (such as the destruction of Pallet Town during the eruption of Mount Moon) or attacked by villains (such as When Rivalen Met Goldilocks).
Finally, one episode of the anime did feature a character die. In A Test of Courage, Max's Alola Forme Miltank suddenly collapsed and died after being subjected to an Extreme Cold Weather Condition. This means that Miltank has now become the first known dead Pokemon in the history of the franchise.