Bacteria are microorganisms that are found widely throughout nature. Bacteria can be pathogenic and cause diseases when entering the body. Their capacity for developing infections and associated pathologies depends on the type of bacteria and its location as well as the resistance (immunity) of the individual. Frequent infectious diseases such as those affecting the respiratory tract, sinuses and ears, are caused by pathogenic bacteria that can normally be present in the pharynx or the nose. In situations in which our defense mechanisms are weakened, for example due to a viral infection (e.g., common cold or flu), bacteria can infect surrounding tissue causing rhinitis, otitis, sinusitis, pharyngitis bronchitis and pneumonia. Bacteria are also a common cause of urinary tract infections, causing cystitis, prostatitis, and pyelonephritis.

There is an enormous diversity of bacteria species grouped in families, genders and strains.

Bacterial vaccines. These vaccines are directed against the pathogenic bacteria causing the infection. They can be made from whole killed/inactivated bacteria, e.g., cholera vaccine, or from live attenuated bacteria, e.g., BCG tuberculosis vaccine. In other instances vaccines are made with a fraction of the bacteria, e.g. pneumococcal polysaccharides, or with the toxin produced by the bacteria, e.g. tetanus vaccine.

Bacterial immunotherapy. The immune system has an innate ability to recognize molecular patterns that occur in the most common microorganisms, including bacteria. For this, the cells of innate immunity have sensors that detect their presence and are activated. Bacterial immunotherapy exploits this fact to enhance the immune response in a nonspecific way, and is the basis of what is known as heterologous immunity. This immunity allows immunization with certain bacteria to acquire resistance against others, and even against unrelated pathogens, e.g. virus.