The advent of genomics should have facilitated the identification of microbial virulence factors, a key objective for vaccine design. When the bacterial pathogen infects the host it expresses a set of genes, a number of them being virulence factors. Among the genes identified by techniques as microarrays, in vivo expression technology, signature-tagged mutagenesis and differential fluorescence induction there are many related to cellular stress, basal metabolism, etc., which cannot be directly involved in virulence, or at least cannot be considered useful candidates to be deleted for designing a live attenuated vaccine. Among the genes disclosed by these methodologies there are a number of hypothetical or unknown proteins. As they can hide some true virulence factors, we have reannotated all of these hypothetical proteins from several respiratory pathogens by a careful and in-depth analysis of each one. Although some of the re-annotations match with functions that can be related to microbial virulence, the identification of virulence factors remains difficult. © 2009 Springer Science+Business Media, LLC.