In the first place, a vast registry of medical-record information on tens of millions of Americans and Canadians, called the Medical Information Bureau (MIB), is available to most insurance companies. Insurance applicants with serious medical conditions, as well as those known to indulge in dangerous hobbies (e.g., skydiving), are almost sure to have records in the MIB. These records are obtained from insurance applications and physical exams. Click here to be taken to MIB's site where, for a modest fee, you can request a copy of your MIB report, if any exists. 

But it's not only insurance company consortiums like MIB that have access to medical records. They're also available to pharmacists, medical records personnel, lab personnel, insurance-company personnel, and many others (in addition, of course, to physicians). However, none of these sources is likely to breach the confidentiality of medical records, even if you (unwisely) ask them to. 

1. Civil records. Particularly useful, if they exist for your subject, are lawsuits, product liability suits, personal injury suits, and malpractice suits. 
2. Workers' Compensation Records. The availability of these records to the public varies from state to state. 
3. Motor Vehicle Records. DMV records may list physical impairments, DUI tickets, and injuries suffered as a result of driving accidents. 
4. News databases. Particularly if your subject is a celebrity, articles describing health problems or accidents may turn up in the news. 
5. Interviews and Surveillance. A private investigator can interview neighbors, business associates, etc. to uncover medical information; or he can conduct surveillance of the individual which may provide clues to his or her health condition. "Dumpster diving" is yet another trick used by some PI's; it involves surreptitiously rummaging through the subject's trash once it's been deposited at the curb, looking for clues like insurance claim forms, etc. This is a surprisingly common practice -- and legal since once trash is dropped at curbside, it becomes public property. (If you have info about yourself you want kept confidential, shred or burn any pertinent documents -- e.g., credit card receipts, telephone bills, etc. -- before tossing them in the trash!) 
6. In most states, companies with more than 15 employees are legally permitted to review the medical histories of job candidates to whom they have made conditional employment offers.