In Hayao Miyazaki's animated film My Neighbor Totoro, the illness of Satsuki and Mei's mother is never directly named, this is a conscious artistic choice, creating an atmosphere of anxiety and uncertainty. However, several conclusions can be made about her condition based on the context.
What was the illness of the heroine of the anime My Neighbor Totoro
Presumably, she suffered from tuberculosis of the spine. For example, the heroine is in a sanatorium outside the city, which is typical for the treatment of this disease in post-war Japan, and the action of the film takes place in the 1950s.
It is also mentioned that her condition is "not dangerous, but requires a long recovery." And this is a typical formulation for tuberculosis at that time.
In addition, Miyazaki often touches on the theme of the fragility of life. In the 1950s, tuberculosis was widespread due to poor nutrition and unsanitary conditions. The girls' father works as a professor, but the family lives modestly - perhaps the mother needs expensive treatment.

The Meaning of the Mother's Illness in the Anime My Neighbor Totoro
The mother's illness reflects the children's fear of losing loved ones - the central theme of the cartoon. And her recovery coincides with the return of Mei after her disappearance - as if Totoro "heals" the whole family.
Miyazaki deliberately avoided medical details in order to preserve the magical atmosphere of the fairy tale, to make the story closer to all children who worry about sick parents and to focus on emotions, not on the disease.
It is interesting that in the early drafts the mother was supposed to die, but the director changed the ending so as not to traumatize young viewers. By the way, the story with the sick parent was very close to the director.
Attachment to his own mother and the trauma of her long illness became one of the key motives of Hayao Miyazaki's work. His mother suffered from tuberculosis of the spine and spent 9 years bedridden, when the future director was only 6-15 years old.