The government studied why so many veterans get ALS. Especially the Gulf vets. They never found a cause, but the statistics showed that there was definitely something service-connected about the disease, so the VA decided that all cases of ALS would be presumed Service Connected, without the vet having to prove a nexus between his disease and his service. So ALS is a "presumptive" illness. You just had to have 90 days active duty and an "other than dishonorable" discharge.
Most illnesses are not presumptives. To get VA to compensate you for, say, Snotty Nose Syndrome (tm), you would have to prove that SNS is somehow disabling or preventing enjoyment of life and that it is "more likely than not" connected or aggravated by your military service. The connection is called a nexus. It would take months or years for VA to research, examine, adjudicate, and rate a claim of SNS.
Let's say you are finally diagnosed with Progressive Muscular Atrophy, an LMN disease. VA has recently made this one a presumptive, but it starts at only 30%. You would receive monthly compensation at the 30% rate. As your condition degrades, you would have to prove that it is now more disabling than it used to be. More appointments, more tests, more paperwork in order to get more compensation. Eventually, you might become rated at 60% SC, at which point you could apply for a P&T rating. As the ratings get higher, you get increased compensation. And, believe it or not, there are higher ratings than 100%.
Fortunately, Uncle Sam does a fairly good job of caring for and compensating people who have been disabled or killed by service to our country. But all the Permanently and Totally disabled vets that I have met have said "the compensation is nice, but I'd rather have my health back."
*** A veteran is someone who, at some time in their life, signed a blank check to the American people, payable "up to and including my life." *** Not many people have what it takes to do that.