Grass and bare legs: Italian soccer ALS cluster
Italian soccer players worldwide have a six times greater chance of getting ALS than the average Italian. This risk is unrelated to general physical activity or doping, because Italian basketball or volleyball players have no increased risk.   Neither is the risk related to just being outside a lot, because Italian cyclists also do not run an extra risk of ALS, according to Raffaele Guariniello, a Turin prosecutor who investigated the early deaths of more than 40 ex-soccer players from ALS. His team could not yet identify a rugby player with ALS, but that was to be expected, since the number of rugby players in Italy is much less than the number of soccer players, so even if rugby players would run the same increased risk of ALS as soccer players, statistically, less than one would be expected to have died of the disease.
Some of the more well-known players who died of ALS or still have it are Armando Segato, Albano Canozza, Lauro Minghelli, Giancarlo Galdiolo, Adriano Lombardi, Stefano Borgonovo, Gianluca Signorini, Giorgio Rognoni and Guido Vincenzi.
Borgonovo said in an interview that he thinks that a genetic flaw lead to his demise, but the evidence contradicts that. He did not suffer from familial ALS but from sporadic ALS - the type that is thought to occur randomly, and not six times more often amongst soccer players.
ALS does not just plague Italian soccer players: Playing soccer is just as risky for Brits: Three British men, carpet fitter George Pearce of Wash Common, electrician Graham Hodgetts of Thatcham and builder and carpenter Sam Brown of Kingsclere all played amateur soccer in the same league on the same soccer pitches at the Newbury and Basingstoke leagues from when they were teenagers until they were in their forties, and in later life died of ALS. Statistically, this is unlikely in the extreme.
It has already been established that merely "being fit" does not raise the risk of acquiring ALS as much as specifically spending a lot of time in nature or on grass. A study published in the American Academy of Neurology found that people who played university sports had a 1.7 times higher risk of developing the disease than those who did not. That is still several times less than the sixfold incidence of ALS amongst Italian soccer players.
There have been suggestions that heading a heavy ball could have initiated a cascade of brain damage leading to ALS, but there is no evidence in medical literature that boxers have a greater risk of developing ALS, making that hypothesis unlikely. And modern soccer balls are much lighter because they are coated to prevent rain water absorption - yet also in recent times, Italian soccer players continued to contract ALS at an alarming rate.
The "physical activity" hypothesis of ALS is based on solid data, but the conclusion that physical activity in general can contribute to causing ALS is unwarranted because it's still a tiny fraction of the physically active that go on to develop ALS. What should be looked at is the type of physical activity that results in the highest rates of ALS. Physically active people usually are active in many ways. They camp and hike as well as play tennis, swim and cycle. Clearly, some activities are much riskier than others. Soccer has been identified as the riskiest sport activity, so far.
What is unique about soccer is that players occasionally cause themselves tiny cuts and abrasions on the grass and dirt when they tackle an opponent. And even the shortest grass harbors ticks, since ticks normally dwell directly on the ground to avoid drying out, until they are ready for a blood meal - only then do they seek a suitable spot to attach to a host.