Although the vast majority of bacteria are harmless or beneficial, quite a few bacteria are pathogenic. Pathogenic bacteria are bacteria that cause bacterial infection. One of the bacterial diseases with highest disease burden is tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which kills about 2 million people a year, mostly in sub-Saharan Africa. Pathogenic bacteria contribute to other globally important diseases, such as pneumonia, which can be caused by bacteria such as Streptococcus and Pseudomonas, and food borne illnesses, which can be caused by bacteria such as Shigella, Campylobacter, and Salmonella. Pathogenic bacteria also cause infections such as tetanus, typhoid fever, diphtheria, syphilis, and leprosy. Koch's postulates are criteria designed to establish a causal relationship between a causative microbe and a disease. Four criteria that were established by Robert Koch to identify the causative agent of a particular disease, these include:

1. 1. The microorganism or other pathogen must be present in all cases of the disease

2. The pathogen can be isolated from the diseased host and grown in pure culture

3. The pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal

4. The pathogen must be reisolated from the new host and shown to be the same as the originally inoculated pathogen.

The changes to the host, manifested as a set of symptoms, may be due to the effect of microbial products such as toxins or the result of the host's immune reactions to the presence of the bacteria. Pain, fever, redness and swelling are common symptoms of bacterial disease.

Fig. 1. Different infection stages of disease causing organisms

A pathogen is a microorganism that is able to cause disease in a plant, animal or insect. Pathogenicity is the ability to produce disease in a host organism. Microbes express their pathogenicity by means of their virulence, a term which refers to the degree of pathogenicity of the microbe. Hence, the determinants of virulence of a pathogen are any of its genetic or biochemical or structural features that enable it to produce disease in a host. The relationship between a host and a pathogen is dynamic. The outcome of such a relationship depends on the virulence of the pathogen and the degree of resistance and susceptibility of the host, due to the effectiveness of the host defense mechanisms.

Two qualities of pathogenic microbes by which they cause disease to the host:

1. Invasiveness is the ability to invade tissues. It consists of colonization, production of extracellular substances which facilitate invasion and ability to bypass or overcome host defense mechanisms.

2. Toxigenesis is the ability to produce toxins. Bacteria may produce two types of toxins called exotoxins and endotoxins. Exotoxins are released from bacterial cells and may act at tissue sites removed from the site of bacterial growth. Endotoxins are cell-associated substance. The bacterial toxins, both soluble and cell-associated, may be transported by blood and lymph and cause cytotoxic effects at tissue sites remote from the original point of invasion or growth.

The diseases can be caused by:

• •  Bacteria:- These one-cell organisms are responsible for illnesses such as strep throat, urinary tract infections and tuberculosis.

•  Viruses:- Even smaller than bacteria, viruses cause a multitude of diseases — ranging from the common cold to AIDS.

•  Fungi:- Many skin diseases, such as ringworm and athlete's foot, are caused by fungi. Other types of fungi can infect your lungs or nervous system.

•  Parasites:- Malaria is caused by a tiny parasite that is transmitted by a mosquito bite. Other parasites may be transmitted to humans from animal feces.