Tyzzer's is a clostrium bacteria that lives in the soil, and only a very small percentage of foals are succeptible to it. It destroys the foal's liver very quickly. I've seen 3 foals with Tyzzer's (2 of ours, about 15 years apart) and they all deteriorated and died within a matter of hours, despite intensive treatment at the vet's.
However, recently here on the forum there was a story about a foal who was diagnosed with Tyzzer's and survived!