Intratumoral application of intact microbes and/or foreing antigens is a promising strategy in anti-cancer immunotherapy since, after the induced immune activation, antigen spread toward tumor-associated antigens and/or neo-antigens can generate an anti-tumor vaccine effect. However, the strong immunosuppressive nature of the tumor microenvironment (TME) has been shown most often to curb the mounting of an effective immune response. By examining the literature, we found evidence that pre-existing immunity to foreign antigens has a crucial role in determining the actual degree of immune activation obtainable. Therefore, assessing or even voluntarily inducing pre-existing immunity to specific foreign antigens can help develop successful immunotherapy. Furthermore, pre-existing immunity to common pathogens developed by previous infection and/or vaccination could be exploited to turn cold tumors into hot ones, a most desired condition for successful checkpoint inhibitor based anti-cancer immunotherapy.