In Poland having health insurance is obligatory. When applying for a visa to Poland and legalising your stay in Poland, you always need to present an insurance coverage document.
The European Health Insurance Card (EHIC) gives you access to necessary medical services for free when you travel temporarily in the European Union as well as to Iceland, Liechtenstein, Norway, Switzerland and the UK (until the end of 2020 due to Brexit) for tourist or work purpose.
It works both ways meaning that anyone covered in Poland by the state-funded NFZ insurance can obtain such a card - for free in the NFZ office competent for the place of residence.
It is important to know, however, that based on EHIC one can use the medical services on the same conditions as residents of a given country. This means that if a certain service is free-of-charge in Poland it may not be so abroad.
When you move your habitual residence to another EU country, Iceland, Liechtenstein, Norway, Switzerland or the UK (before the end of 2020) you should obtain an S1 document instead of using the EHIC to receive medical care in the new country of residence. S1 entitles you to healthcare if you don't live in the country where you are insured. It may be useful for posted workers, cross-border workers, pensioners and their family members.
In order to obtain the S1 document in Poland, you need to apply to NFZ.
The document should be registered in a health insurance institution in the country of the new habitual residence.
Persons insured in the National Health Fund (NFZ) can use state-funded healthcare services that are mostly free of charge, with rare exceptions for example in the case of some dentistry services.