A Long Day’s Journey into the Night is a play by Eugene O’Neill, and it premiered in 1956. It follows one day in the life of the Tyrone family, which explains the title.
Illness is a major theme in this play. Indeed, one could definitely argue that illness is used as a metaphor for the bad, poisonous atmosphere within the family.
During the course of the play, it becomes very quickly apparent how bad the relationship is between the various family members. The family is not as close as one would expect. The family members are very distant from each other, as if they are scared of catching a disease from the others. As a result, they all have turn to drugs of some sort in order to be able to cope with this "disease."
Ironically, there is a lot of illness within the family, and these illnesses are used as metaphors for the bad relationships between the individual family members. For example, Mary, the mother, has been suffering from morphine addiction ever since giving birth. This could be seen as a metaphor for the challenges of raising her children. She is unable to lose this addiction, just as she was unable to get along better with her children when they grew up.
James, the father, is addicted to alcohol, which again could be seen as a metaphor for the fact that he is unhappy with his life and tries to forget his sorrows through the use of alcohol. Through his addiction, James is able to escape his mundane life, at least temporarily.
Edmund suffers from tuberculosis, and because of this, he will need to spend some time in a sanatorium. Tuberculosis is an illness of the lungs. Therefore, it could be seen as a metaphor for how Edmund is unable to lead a proper life while being in his family. It is as if being surrounded by his family stops him from breathing freely and from being himself. Having to go to a sanatorium could therefore be seen as the opportunity to finally leave his family and become free—the remedy he has been waiting for.