Unfortunately, this simple question has a fairly complicated answer. The answer is: it depends. Firstly, it depends on whether you are a US resident traveling outside the US or a visitor to the US. US residents can purchase trip cancellation insurance that can cover pre-existing conditions and even plans that are travel medical only without cancellation often are more generous in their coverage of pre-existing conditions. These plans may have specific requirements in order to waive the usual exclusion of coverage for pre-existing conditions (like purchase them within a certain number of days of your initial trip deposit or that you have a primary domestic insurance plan in the US), so you should check these details carefully if this is important to you.
Non-US residents or US residents traveling into the US have much more limited options to insure their pre-existing conditions are covered. Some plans offer coverage for "acute onset" of pre-existing conditions. However, this is misleading, since a careful look at their definition excludes chronic or congenital disorders from such coverage. Chronic illnesses are those that worsen over time and congenital disorders are issues one is born with. They exclude emergencies due to chronic illness because they will say that is not "acute" it is simply a worsening of your condition. There are only two policies we know of that offer coverage for pre-existing medical conditions. They are INF Premier and INF Elite plans.
Firstly, they define pre-existing conditions in the following way: A pre-existing condition is a condition for which the insured received treatment or took medication for in the previous 12 months before the start of the policy. If the medication dosage has not changed and is taken as a maintenance medication, this would NOT count as a flare up or treatment of the condition. In that case, very stable conditions that have no flare ups in the past 12 months are not even excluded as pre-existing conditions. Conditions that have had flare ups or doctor recommended treatment or change in medication, are still covered by the plans but with a higher deductible and a lower maximum.
INF Elite (also INF Elite 90) are comprehensive plans offering fairly complete coverage. INF Premier is a fixed plan. Find details on these plans here.
The above plans must be purchased for at least 90 days and can be quite pricey for seniors, but they offer the only real coverage for chronic conditions.
The other option, of course, is to take a plans with little ($1K) or no coverage for pre-existing conditions and take a risk that that condition will not flare up during the trip.
