Insurance coverage for preexisting conditions like the one below is quite common. A candid and precise answer is, ‘if the current health insurance in traveler’s country of origin not cover for the person’s preexisting conditions during travel overseas, then no company in the world is willing to take the full risk of insuring an already sick person’.
“Both my parents have medical issues but would like to travel from UK to US to visit me. They want to take out health insurance in the event anything were to happen but they are worried it won’t be valid due to pre-existing medical issues. Any suggestions as to who to take insurance out with and where they would stand with regards to doctor visits or hospital visits if for example- heart problems or blood clots occurred and they already have disorders that may possibly cause them.”
However there are some plans that offer TOKEN coverage for pre-existing conditions and you can review the options at http://www.nriol.net/visitor-medical-insurance/