Dead or alive: how the immune system detects microbial viability

•Immune cells discriminate between viable and dead microbial matter.•Recognition of microbial viability represents a highly conserved innate immune checkpoint with wide-ranging impact on immune responses.•Microbial RNA represents a versatile molecular signal of microbial viability.•TLR8 plays a key role in the recognition of microbial RNA and in the initiation of protective immunity. Immune detection of microbial viability is increasingly recognized as a potent driver of innate and adaptive immune responses. Here we describe recent mechanistic insights into the process of how the immune system discriminates between viable and non-viable microbial matter. Accumulating evidence suggests a key role for microbial RNA as a widely conserved viability associated PAMP (vita-PAMP) and a molecular signal of increased infectious threat. Toll-like receptor 8 (TLR8) has recently emerged as a critical sensor for viable bacteria, ssRNA viruses, and archaea in human antigen presenting cells (APC). We discuss the role of microbial RNA, and other potential vita-PAMPs in antimicrobial immunity and vaccine responses.