There is no question that proof is required that “some act or phase of the employment was a causative factor” in the occurrence of heart attack or stroke. But it is not necessary to prove that the employment was the sole causative factor, or even that it was the principal causative factor, but only that it was a causative factor. The mere fact that an employee is at work when a heart attack occurs is insufficient to justify an award absent proof of a causal connection between the employment and the disability. A pre-existing heart condition does not preclude a finding of compensability if the Commission may legitimately infer from the evidence that the employee’s work duties were a causative factor in the heart attack or exacerbation of coronary disease.