For example many CPSU members were formally members of the social democratic party, almost all Eastern Bloc communist parties were mergers of the preexisting communist parties with social democratic parties, and the Korean Social Democratic Party is one of the few opposition parties in the DPRK
like others have said, social democrats and communists have a shared history due to word choice. the Bolsheviks (russian social democratic labor party) called their party that after imitating the success of the SPD, social democrat basically was the same as communist in the 19th century, but communist was more associated with 1848/1870 than the SPD. Lenin like others said, brought back the term to counter the revisionist SPD treachery in 1914.