There is no official answer to this, just some fans' opinions but one fan believes that because she was separated from her chosen Digimon partner the separation caused her to fall ill. Another theory is that the mystery voice from the Digital World that can only communicate with Kari was in contact with her at the time and caused her to fall ill.
Kari Kamiya is a character in the first and second seasons of Digimon.
No, Kari never appears in Digimon Tamers.
Her first chonological appearence is in the first Digimon movie. If you are referring to her first appearence in a proper Digimon season it happened in the 21st episode of Adventure.
No, Kari and Willis do not fall in love with one another in Digimon.
Kari's Crest is the Crest of Light.
No Yggdrasil was not the mysterious entity that spoke through Kari in one episode of the first season of Digimon Adventure. Yggdrasil is from Digimon Data Squad which is a separate continuity from Digimon Adventure and therefore doesn't exist in the Digimon Adventure continuity.
Kari's crest is the crest of light.
Episode 21 of Digimon Adventure.
Tai
Lara Jill Miller
Kari got the flu from having a weak immune system. Kari died because they did not treat the flu in time.
Kari, Tai's little sister.