Leigh was an extremely ambitious person and throughout her life her body paid an enormous price for that. She was so disciplined and determined that she could work herself out to the brink of falling unconscious, paying no attention to red flags of all sorts that the actress' organism was signalling to her.
Her mind was such a dominant force that even in the period of her life when she struggled the most with her mental health, she was fully capable to give perfect performances only to go behind a curtain and break apart when she was sure the audience won't see it.
In 1937, Leigh fell seriously ill for the first time. She was rehearsing for the play Henry VIII. The weather was terrible and she was partially exposed to it. She caught influenza and could not shake it off, but did not exactly take pity on her body to try and do so. On top of that, she was smoking heavily at the time.
During the making of Caesar and Cleopatra, the star found out that she is pregnant. The doctor told her that she has to stop filming for the time of pregnancy for the good of the baby. She didn't listen and soon paid the ultimate price for it.
The shooting weakened her body and she fell ill. On August 29, while she was shooting a scene in Memphis Palace where Cleopatra runs to declare to Caesar that she is the real queen, she slipped and fell. Two days later, she miscarried.
It's worth noting that different Vivien Leigh biographies point to two different causes of that miscarriage. One is overworking, the other is slipping. What was the real cause is of course unknown, but it's fair to assume that both contributed.
Since that miscarriage, the actress began feeling and looking worse both physically and mentally. At one point, she felt so bad that she quickly went to her doctor. He diagnosed her with pulmonary tuberculosis.
After getting it confirmed with another doctor, Leigh decided to play until the end of the month and then begin treatment at University College Hospital.
Again, this shows how driven and (in result) irresponsible she was. When left untreated, pulmonary tuberculosis can lead to life-threatening damage to many organs.
Fortunately, both doctors did not give up and urged her to stop working immediately until she finally did. One of the rooms in Leigh and Olivier's freshly-bought home Notley Abbey was converted to a mini health resort. In that room she spent majority of time during the nine months she was coming back to health.
At first, she did not like the room and felt trapped and hopeless. With time, she got used to it and her physical health began to improve. Mentally, she started feeling better too, but psychotic episodes were still occuring.
In 1952, lung problems came back, invited once again by working too much, smoking so many cigarettes and this time also drinking heavily (which can also cause many lung diseases). To battle her condition, the actress took medicine which shouldn't be mixed with alcohol, and yet she was drinking like never before.
She somehow survived it, but certainly not in perfect health.
At the beginning of the 1960s, pulmonary tuberculosis came back to haunt her. This time, she was living more proper - no drinking, smoking little. Despite that, she felt terrible and lost alarming amount of weight.
In 1966, she was giving the last round of performances of her theatrical career when she played Anna Petrovna in Ivanov (not knowing it, of course). The star looked so weak that people around her were worried that she won't be able to physically make it until the last curtain falls. In a typical Vivien Leigh fashion, she forced herself to finish it properly... and her body suffered as a result.
Despite so many situations when neglecting her body had costed her so much, the actress continued being reckless with it and this is what ultimately caused her her life.
While she was taking part in rehearsals for the play A Delicate Balance, tuberculosis reared its ugly head again.
When her doctors discovered the relapse, they urged her to stop working immediately and going to a hospital. She felt that doing so at home will be enough. Suggested recuperation time was three months and during that time she was told to stay in bed whenever she can, avoid contact with people, avoid opening windows too much etc.
Jack Merivale, Vivien Leigh's partner at the time, had a habit of calling her before entering the stage. One time, her voice was so alarming that right after the play he got home as quick as he possibly could.
When the actor got there, he found out that his love is sleeping, so he went down to make something to eat. When he came back up, he discovered her on the ground, with face down, dead.
The cause of death was pulmonary tuberculosis - fluids gathered in her body because of the disease found their way to the lungs.
Laurence Olivier was informed quickly and soon appeared there. He left after approximately 30 minutes, only saying "no comment" to dozens of reporters already gathered in front of the house.
The funeral took place in Royal Parish Church of St. Martin-in-the-Fields (official) on August 15, 1967. Her wishes were to be cremated, which was done in Golders Green Crematorium - the first crematorium in London history.
As ordered, her ashes were scattered at Tickerage Mill's beautiful pond.