It’s mandatory for all individuals that are living in Belgium, not for expats because their company will provide them.

Its important to have to cover up doctor visits, some medicines and more.

1. PARTENA
2. Christian Mutuality
3. Socialist Mutuality (FLANDERS)
4. https://www.oz.be/
5. https://www.lm.be/Pages/defaultNL.aspx
6. Complete List https://www.riziv.fgov.be/nl/professionals/andere-professionals/ziekenfonds/Paginas/contacteer-ziekenfondsen.aspx

Please check others too, These are used across in Belgium.