Stephen Hawking suffers from amyotrophic lateral sclerosis, which is more commonly referred to as Lou Gehrig's disease or ALS. Although he received his diagnosis at age 21, the severity of his symptoms progressed gradually over the course of decades. Before he lost most of his lower body function because of his illness, he was able to produce three children.
Amyotrophic lateral sclerosis is a neurodegenerative disease that causes nerve cells in both the brain and spinal cord to gradually perish, which inhibits the brain's ability to control movement of muscles. However, as was the case for Stephen Hawking, the progression of the nerve cells' death can be quite slow. Paralysis typically only occurs in the later stages of ALS.
Although he was having difficulty walking and was experiencing major slurring of his speech, Hawking got married in 1965 and had his first son in 1967. In the ensuing years, his physical function began to decline even more, to the point where he was dependent on crutches and was no longer able to write. His second child, a daughter, was born around this time in 1970. Almost a decade later, over the course of which Hawking's speech and motor function continued to slowly deteriorate, Hawking's third and final child, another son, was born in 1979.
As of today, his condition has caused almost full paralysis and some difficulty breathing, which are consistent with the late stages of ALS.