Lawn disease transmission?

I was reading something about lawn diseases being transmitted at another lawn care forum called Lawn cafe. It was mentioned that when a lawn care company mows your grass at one customer's house, they then can carry a lawn disease to your lawn. Is this true and what can you do to prevent this? I wouldn't be able to trust someone giving their word that it won't happen.

Here's the link... http://www.lawncafe.com/t14368-have-you-heard-of-this-maybe-i-am-crazy.html

Without reading the link ... YES! 

Of course, your lawn is already covered with thousands of diseases - your plant life is just tough enough to keep them at bay.  Or not!

Take a look at how 2-4D works.  It emulates the plant hormone that causes growth.  So you spray it on a plant and the plant grows until it uses up all of its energy reserves.  Now that it is weak, the diseases that are everywhere around it can move in.  Or glyphosate (roundup):  it is like plant AIDS.  It turns off the plant's immune system so that all the disease that are lying arround kill the plant.

So you already have heaps of disease on your lawn.  BUT!  I suppose there could be some super disease an that other lawn ....  and it can easily be transferred by foot traffic and equipment.  But I suspect that there isn't much on any one lawn that isn't on all other lawns.  And if your turf is healthy - it should easily hold up to most problems.