What causes the death of someone with lou gehrig's disease?

Mostly infection. When the disease affects motor nerve cells at base of brain, causing bulbar problems, the pt loses the ability to swallow, and breathe, with the need for tracheostomy, and perhaps a peg feeding device. This is a real setup for infection, and pneumonia is not an uncommon end to this dismal and tragic disease.
Weakness. Most patients with lou gehrig's disease die because they get so weak they can't breathe adequately. Patients who get weak enough to be in the bed get infections such as urinary tract infections, pneumonia and bed sores.
Death by paralysis. Amyotrophic lateral sclerosis is also known as lou gehrig's disease. Als is a progressive disease affecting nerve cells in the brain and spinal cord. The nerve cells that control muscle movements degenerate and eventually die. The patient worsens as more nerve cells die. Finally, the patient dies from being unable to move his muscles.