Illness, from minor colds to chronic diseases, creeps into people's lives whether they expect it or not. Many anime characters go through their entire story without ever so much as a sniffle, but anime series about illness offer engaging storylines that focus on one of life's universal struggles.
Anime in which a character is sick can create drama by introducing high stakes. In Full Moon o Sagashite, Mitsuki Koyama could die if she doesn't treat her throat cancer, but if she does address it, she risks losing her voice and her dreams of singing. It can also create an opportunity for characters to come together - consider Looking Up at the Half-Moon, in which Yuuichi Ezaki and Rika Akiba meet and fall in love while confined to a hospital.
Not every anime that includes illness centers entirely around the disease. Sometimes, anime series with an ill character use disease to tell a much larger story. Dragon Ball Z is an action-oriented fighting anime, but there's a point where a character nearly dies of a heart virus, only to be saved by medication from the future. Brace yourself, because some of these extremely sad anime do not hold back.
Your Lie in April deals with some pretty heavy subject matter, focusing on young musical prodigies forced into adulthood by childhood trauma and chronic illness. Kaori Miyazono, a talented violinist who appears full of life, secretly has a deadly disease. The exact illness is never specified, but fans speculate it may be Friedreich's ataxia or autoimmune hemolytic anemia.
Kaori's illness not only causes her to miss out on pursuing her musical career, but also pushes her to hide her romantic feelings for protagonist Kousei Arima. She and Kousei face difficulties unimaginable to many adults, making Your Lie in April an extremely emotional watch.
Since birth, Nagisa Furukawa has suffered from an unnamed hereditary illness that causes repeated bouts of fever, weakness, and pain. Throughout Clannad, she faces repeated attacks of this illness, which cause her to miss school and other social experiences. In Clannad: After Story, it becomes a much larger problem, leading to her death after giving birth to her daughter, Ushio.
Clannad does something with Nagisa's plot that few anime do. When Nagisa expresses concern that she burdens her parents with her problems and prevents them from achieving their dreams, they loudly declare she has done no such thing, and their job is to take care of her and watch her achieve her dreams. It's not uncommon for people with a chronic illness or disability to feel like a burden, so it's refreshing to see Nagisa's parents explicitly reject such fears.
Fullmetal Alchemist: Brotherhood doesn't exactly dwell on illness as a plot point, but one character's health sparks the protagonist's primary goal. Trisha Elric dies after contracting an unnamed disease some fans suspect to be tuberculosis, leaving her two young sons orphaned and hellbent on bringing her back. To do this, they learn as much alchemy as they possibly can and attempt to perform a human transmutation. This results in Ed losing an arm and a leg, and Al losing his entire body.
Trisha's illness isn't the only one that resonates throughout the series. Ed endures physical pain as a result of his loss of limbs, and his teacher Izumi Curtis permanently damaged her organs trying her own human transmutation, which causes her to cough up blood and have frequent dizzy spells. Instead of merely portraying someone struggling with illness, FMA:B also shows how life changes after you lose someone to disease.
While most of Sword Art Online doesn't focus on illness, the second half of the second season explores the possibilities of virtual reality as a form of therapy. The Medicuboid, a machine that enables a severely ill person to enter virtual reality on a near-continuous basis, allows a patient to rest, receive treatment, escape from their pain, and pursue a fuller life than the one their own body could.
Yuuki Konno is dying of AIDS, but the Medicuboid alleviates the process. It provides her the opportunity to interact with other terminally ill people who share her experience, but with whom she can't personally interact due to her compromised immune system. Not only does she meet kindred spirits, but Yuuki also gets to go on adventures and forms a genuine bond with one of the show's protagonists, Asuna.