Horizontal gene transfer has a tremendous impact on the genome plasticity, adaptation and evolution of bacteria. Horizontally transferred mobile genetic elements are involved in the dissemination of antibiotic resistance and virulence genes, thus contributing to the emergence of novel "superbugs". This review provides update on various mechanisms of horizontal gene transfer and examines how horizontal gene transfer contributes to the evolution of pathogenic bacteria. Special focus is paid to the role horizontal gene transfer plays in pathogenicity of the emerging human pathogens: hypervirulent Clostridium difficile and Escherichia coli (including the most recent haemolytic uraemic syndrome outbreak strain) and methicillin-resistant Staphylococcus aureus (MRSA), which have been associated with largest outbreaks of infection recently.