Instituto Mexicano del Seguro Social, IMSS, is Mexico's Social Security Institute. If you are working for a public or private sector employer, you should be automatically enrolled in IMSS. If you are retired or not working and wish to enroll in IMSS, you may be eligible to pay into the system. However, visa status (permanent resident visas are preferred, temporary resident visas are sometimes acceptable), pre-existing conditions and age are all factors in eligibility for IMSS. Eligible expats typically pay $300-400 per year for IMSS coverage.
"Some expats have been able to qualify for Mexico's IMSS (socialized medical coverage). What I've observed is that 'little medical problems' are often not worth taking to an IMSS clinic at all. Folks will just go to a local doctor and pay 200 or 300 pesos ($10 to 15 US) to a local general practitioner, and typically 400 to 600 ($20 to 30 US) pesos for a high-end specialist in a major city. When a friend contracted a rare illness however, her IMSS case was 'bumped up' the system, and she was cared for in a leading Mexico City hospital. (costs quoted here adjusted for Feb 2017 exchange rate)," one expat elaborated.