In this talk we will present cutting-edge technologies to decipher the infection processes at the single-cell RNA-seq. While transcriptomics has unveil cell-to-cell heterogeneity with unprecedented resolution, understading cellular transitions remain a challenge. Here we will introduce scSLAM-seq, a technology that combines RNA metabolic labelling, nucleotide conversion and scRNA-seq to reveal the onset of viral infections. In a second part of the talk we will introduce a novel technology to track the transcriptome of individual bacteria and potentially in the future the transcriptome of complex microbial communities. Finally we also summarize our latest efforts to respond to the current COVID-19 crisis.