Bacteria are prokaryotic, single celled organisms that are the most populous form of life on Earth. In fact, there are more bacterial cells in and on the human body than there are human cells. There are literally untold numbers of different bacteria on the planet. The vast majority of bacteria that humans are in contact with are harmless due to our immune systems. Most that are dangerous can be treated with basic antibiotics. But there are some bacteria called pathogenic bacteria that are very dangerous to humans. It's really impossible to say that there is one single most dangerous bacteria to humans because there are different factors to consider. There are many bacteria that were very harmful to humans in the past but modern treatments and vaccinations have rendered them largely defeated in the first world. But some very dangerous bacteria to humans include Myobacterium tuberculosis (this causes tuberculosis), Streptococcus (causes pneumonia), Salmonella (causes food-borne illness), and multi-drug resistant Staphylococcus (flesh eating disease).