Thomas, who played for the Vikings from 1995-2001, was diagnosed with amyotrophic lateral sclerosis – better known as ALS or Lou Gehrig's disease – in 2004.
Five years later, the Pioneer Press wrote about his slow decline; how he'd gone from a hard-hitting hawkish safety to a man who relied on a ventilator and feeding tube to continue living, and was no longer able to utter a word.
ALS, according to the ALS Association, is a "progressive neurodegenerative disease that affects nerve cells in the brain and the spinal cord."
Essentially, the brain loses its ability to control the body's muscles, leading to a slow deterioration and eventually death.
In August of this year, The Advertiser (from Lafayette, Louisiana) wrote about Thomas' continued struggle with the disease.
Thomas was 140 pounds at that point – he weighed in at 225 during his playing days, ESPN notes – and unable to move even a muscle, his former Crowley High coach, Lewis Cook, told the paper.
Thomas played for the Vikings from 1995 through 2001, racking up 350 tackles, 22 interceptions and seven passes defended during that time. Nine of those picks came in his rookie year, which led the league that season.