What to do when your calf develops diarrhoea

Dear doctor, thanks for the informative articles in Smart Harvest. I started dairy farming four years ago but on a wrong footing when my two in-calf delivered two days apart and I lost the two calves to diarrhoea. I never knew diarrhoea is that fatal in calves. I called in a vet who did post-mortem and later told me my calves had all succumbed to E.Coli diarrhoea. Please educate me on diarrhoea in calves. John Cheruiyot, Marigat