The petitioner filed a claim with the Department of Labor and Industries (Department), alleging that she had contracted hepatitis as a result of her employment as a registered nurse in an intensive care unit at the respondent medical center. When the Department rejected her claim, she appealed to the Board of Industrial Insurance Appeals (Board). A hearing examiner heard the petitioner's evidence, consisting of her testimony and that of two medical witnesses, and granted the respondent's motion to dismiss, ruling that the petitioner had not proved by a preponderance of the evidence that the disease from which she suffered rose naturally and proximately out of her employment. Reviewing that ruling, the Board reversed and allowed the claim. Upon the respondent's appeal to the Superior Court, the jury upheld the decision of the Board. A further appeal to the Court of Appeals, Division Three, resulted in a reversal of the judgment on the verdict. 20 Wn. App. 285, 579 P.2d 412 (1978).
The question before the court is whether the medical testimony was sufficient to sustain a finding by the Board and by the jury that the petitioner's hepatitis was attributable to her employment. We answer in the affirmative.
*633Medical testimony offered by the petitioner showed that hepatitis is a disease which is transmitted by contact with body fluids, including blood, and wastes of a person who either has the disease or is a carrier of it. A carrier may display no symptoms of the disease. There is a greater probability that a nurse working in a hospital, and particularly in an intensive care unit, will contract the disease than there is that a person employed in some other occupation will do so.
One of the doctors was willing to state unequivocally, in answer to a hypothetical question, that the odds were overwhelmingly in favor of the petitioner's having contracted the disease at the medical center. However, the doctor was asked to assume, among other facts, that another nurse at the center had hepatitis. This fact was not proved at the hearing, and while the Board did not discredit the doctor's opinion because of it, the Superior Court granted the respondent's motion to strike that portion of his testimony and it was not submitted to the jury.
The other doctor, while agreeing that a nurse in the petitioner's position is exposed to a greater risk of the disease than persons who do not work in such an environment, refused to give a direct opinion as to the probability that she contracted the disease at the center.1
Both doctors agreed that it is virtually impossible to pinpoint the precise source of a given case of hepatitis, since carriers may not display symptoms of the disease. One of the doctors stated that it is rare that a person suffering from the disease will be hospitalized, since treatment can be administered adequately outside that environment. From this testimony, it can be inferred that the disease is *634transmitted in hospitals primarily by carriers, rather than by persons displaying symptoms.
The petitioner's testimony showed that she had been employed at the center for more than the normal period of incubation; that her duties required daily handling of the fluids and excrements of patients; that on one occasion she had been bitten by a patient; and that the handling of needles and other medical equipment resulted in occasional small cuts and punctures on the hands. Her evenings were usually spent at home, where she lived a quiet life alone, and her attendance at public functions was confined to an occasional movie. She knew of no exposure to the disease which she might have had outside the hospital.
The Board attached no significance to the alleged but unproven fact that another nurse had the disease, focusing its attention instead on the evidence that hepatitis can be transmitted by a carrier who does not have an active case of the disease and that the risk of contracting the disease is greater in a hospital than outside, a fact which the Board observed should be "rather obvious." As we have previously noted, the jury's verdict affirming the Board was reached without considering the opinion of the doctor who was asked to assume that another nurse had the disease.
In the Court of Appeals, the respondent argued that an award cannot be sustained in the absence of a medical opinion that the disease or condition probably, or more likely than not, arose out of the claimant's employment. That court, while acknowledging that in a proper case circumstantial evidence may be considered in conjunction with medical testimony in arriving at the source of a disease, concluded that the claim should be denied because the petitioner had failed to show that any person with whom she came in contact at the hospital had the disease of hepatitis.
In reaching this conclusion, the Court of Appeals quite evidently overlooked the testimony that a carrier of the disease may be undetectable as such. If the petitioner contracted the disease from such a person, the fact that there *635was at the hospital another person who had the disease and manifested the symptoms would be entirely irrelevant. While such proof might afford room to infer that there was an increased opportunity to contract the disease, the medical testimony which was presented was sufficient to support an inference that the petitioner did come in contact with a carrier at the hospital, even though that carrier was not recognizable as such.
It must be kept in mind that the industrial insurance act, while it changes the common law, is remedial in nature and is to be liberally applied to achieve its purpose of providing compensation to all covered persons injured in their employment. That purpose is set forth in RCW 51.04-.010.
Furthermore, the petitioner's claim reached the Court of Appeals supported by two presumptions, that the findings and decision of the Board, not having been rejected by the jury, were correct (RCW 51.52.115), and that the verdict was correct.
It is generally held that the evidence in a workmen's compensation case will be regarded as sufficient where the circumstances shown tend to establish the ultimate facts in issue or provide a basis from which they reasonably may be inferred. 82 Am. Jur. 2d Workmen's Compensation § 533 (1976). As is revealed in 1 A. Larson, The Law of Workmen's Compensation § 8.50 (1978), a number of courts have allowed recoveries where the evidence showed that the claimant's working environment involved an increased risk of contraction of a disease. In Roe v. Boise Grocery Co., 53 Idaho 82, 21 P.2d 910 (1933), compensation was awarded for the death from Rocky Mountain spotted fever of a salesman whose duties required frequent trips through territory infested with wood ticks, although there was also evidence of two personal trips into that territory. The California Appellate Court allowed recovery for the death of an employee who had been sent to Peru during a typhoid epidemic to which foreigners would be peculiarly susceptible, and had contracted the disease. Fidelity & Cas. Co. v. *636Industrial Accident Comm'n, 84 Cal. App. 506, 258 P. 698 (1927). And in Lothrop v. Hamilton Wright Organizations, Inc., 45 App. Div. 2d 784, 356 N.Y.S.2d 730 (1974), an employee sent into Bolivia to take photographs, who contracted infectious viral hepatitis upon his return, was held to have contracted the disease as a result of his exposure to deplorable sanitary conditions. Even though the etiology of polio was obscure, a nurse who worked in a polio ward and became ill with the disease was allowed to recover upon a showing that the incidence of the disease is higher among nurses and others constantly in contact with the disease. Industrial Comm'n v. Corwin Hosp., 126 Colo. 358, 250 P.2d 135 (1952).
While some courts have taken a more conservative approach, as Larson's and an annotation in 20 A.L.R. 4 (1922), disclose, we believe that the spirit and intent of the industrial insurance act is better served by the liberal philosophy expressed by these courts.
We are mindful that medical testimony forms a vital part of a claimant's proof, particularly where it involves matters which are beyond the knowledge and understanding of laymen. Accordingly, we have adhered to the rule that the causal connection between a claimant's physical condition and his employment must be established by such testimony. Ehman v. Department of Labor & Indus., 33 Wn.2d 584, 206 P.2d 787 (1949); Parr v. Department of Labor & Indus., 46 Wn.2d 144, 278 P.2d 666 (1955). We have also said that such medical testimony must show a greater probability of a causal relation and not a mere possibility. Seattle-Tacoma Shipbuilding Co. v. Department of Labor & Indus., 26 Wn.2d 233, 173 P.2d 786 (1946).
Of course, such causation is usually shown by the eliciting of medical opinions upon the particular issue, but we do not read our cases as requiring in every case and under any and all circumstances the production of a medical opinion upon the ultimate issue. It is sufficient if the medical testimony shows the causal connection. If, from the medical testimony given and the facts and circumstances proven by *637other evidence, a reasonable person can infer that the causal connection exists, we know of no principle which would forbid the drawing of that inference. See Seattle-Tacoma Shipbuilding Co. v. Department of Labor & Indus., supra at 242.
Here the medical testimony showed that there is generally a greater probability that a person in the petitioner's employment will contract hepatitis than there is that someone in another employment will do so. In other words, there is a greater risk of getting the disease when one is employed in a hospital. The Board and the jury were entitled to consider this evidence in conjunction with the petitioner's uncontradicted and undiscredited testimony that none of her other activities involved such exposure, and to reach the conclusion that, more likely than not, the disease resulted from contact with a carrier in the hospital.
The decision of the Court of Appeals is reversed and the judgment on the verdict is reinstated.
Utter, C.J., Wright, Dolliver, and Hicks, JJ., and Ryan, J. Pro Tern., concur.As the Superior Court judge noted, this refusal appears to have been inspired by a misunderstanding of the question, since the doctor's answer was that he could not determine whether she got the disease inside or outside the hospital. In other words, he appears to have thought that he was being asked to state with certainty what the source of her disease was, when in fact he was being asked to answer in terms of probabilities.