The occupational diseases defined in Section 52-3-33 N.M. Stat. Ann. shall be deemed to arise out of the employment only if there is a direct causal connection between the conditions under which the work is performed and the occupational disease and which can be seen to have followed as a natural incident of the work as a result of the exposure occasioned by the nature of the employment and which can be fairly traced to the employment as the proximate cause. The disease must be incidental to the character of the business and not independent of the relation of employer and employee. The disease need not have been foreseen or expected but after its contraction must appear to have had its origin in a risk connected with the employment and to have flowed from that source as a natural consequence. In all cases where the defendant denies that an alleged occupational disease is the material and direct result of the conditions under which work was performed, the worker must establish that causal connection as a medical probability by medical expert testimony. No award of compensation benefits shall be based on speculation or on expert testimony that as a medical possibility the causal connection exists.