Temporary travel insurance can usually be arranged at the time of the booking of a trip to cover exactly the duration of that trip, or a "multi-trip" policy can cover an unlimited number of trips within a set time frame. Coverage varies, and can be purchased to include higher risk items such as "winter sports".
3rd party supplier insolvency (e.g. the hotel or airline to which you made non-refundable pre-payments has gone into administration)
Acute on-set of pre-existing conditions - This term is very different from Pre-existing conditions. It means a sudden and unexpected occurrence of pre-existing medical conditions without any prior warning from a medical professional. Travel insurance companies, especially in USA, do not offer coverage for pre-existing coverage but they do offer coverage for acute on-set of pre-existing conditions. If you have pre-existing conditions, make sure to get this coverage to protect yourself against any emergency situation that arises due to pre-existing conditions in spite of taking care.
Pre-existing medical conditions but a lot of companies now cover for "acute on-set of pre-existing conditions"
War or terrorism - but some plans may cover this risk, and some do cover for acts of terrorism
Injury or illness caused by alcohol or drug use
Usually, the insurers cover pregnancy related expenses, if the travel occurs within the first trimester. After that, insurance coverage varies from insurer to insurer.
Travel insurance can also provide helpful services, often 24 hours a day, 7 days a week that can include concierge services and emergency travel assistance.