Opportunistic infections that develop with AIDS are infections by organisms that do not cause disease in people with healthy immune systems. Both the HIV infection and the opportunistic infections and cancers produce the symptoms of AIDS.36[36]   Pneumonia caused by the fungus Pneumocystis carinii is a common and recurring opportunistic infection in people with AIDS, and is the first opportunistic infection to develop. Tuberculosis is more frequent and deadlier in people who have HIV infection than in those who do not, and is difficult to treat if the strain of the tuberculosis is resistant to antibiotics. Another mycobacterium, Mycobacterium avium complex, is a common cause of fever, weight loss, and diarrhea in people with the advanced disease.37[37]   AIDS is not listed as an occupational disease both under the POEA-SEC and the ECC Rules. Thus, the claimant bears the burden of reasonably proving the relationship between the work of the deceased and AIDS, or that the risk of contracting AIDS was increased by the working conditions of the deceased.   In the present case, we do not find Eduardo’s AIDS to have been work-related. Records have shown that it was a pre-existing illness that Eduardo did not disclose during his PEME with the respondents’ medical testing center.   The evidence reveals that Eduardo had undergone a previous PEME on October 29, 1997 (or two years before his deployment with the respondents) as a prerequisite for his employment with another agency – Southfield Agencies (Southfield). The PEME was conducted by the PROBE Polyclinic and Diagnostic Center (PROBE), Southfield’s designated testing center. Dr. Laura S. Gonzales, the examining physician, found Eduardo positive for HIV, and declared him unfit for sea duty.38[38] Eduardo was then advised to proceed to the Department of Health’s National Reference Testing Center for HIV Testing for further examination and tests. The National Reference Testing Center for HIV Testing confirmed the findings of PROBE, and declared Eduardo to be HIV positive.39[39] 36[36] Id. at 929-930. 37[37] Id. at 930. 38[38] CA rollo, p. 91. 39[39] Id. at 92.