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Infectious Diseases

Author(s): Christine Herrmann, PhD

How Infectious Agents Cause Disease

Many microbes produce toxins or enzymes that damage host tissue. Bacteria produce two groups of toxins: endotoxins and exotoxins. Endotoxins are part of the cell wall of some types of bacteria and are released into host tissues when the bacteria divide or die. Exotoxins are secreted by bacteria into host tissues; they are sufficient to make a person ill even in the absence of the bacterial organisms.

There are seven different types of the exotoxin produced by the organism that causes botulism (Clostridium botulinum). Types A, B, E, and F cause a severe type of food poisoning, producing one of the most poisonous substances known to mankind. Types C, D, and E cause illness in mammals, birds, and fish. Botox, a pharmaceutical, is a purified and diluted A neurotoxin used for clinical and cosmetic purposes (e.g., to remove wrinkles by paralyzing forehead muscles for several months). Viruses must invade a cell directly in order to reproduce and in the process disrupt normal host cell functions. In many cases, the assembly and release of new virus particles causes the cell to lyse or rupture, releasing the new virus particles which can go on to infect more cells. Fungi and protozoa also can release toxins and enzymes that destroy host tissues. Some protozoa, such as the parasite that causes malaria, directly invade host cells.

Many signs and symptoms of disease are brought on by the host’s immune system in response to pathogen invasion. Fever, for example, is an attempt by your body to kill invading microbes that are sensitive to changes in temperature. Sneezing, coughing, vomiting, and diarrhea all are efforts by the body to rid itself of pathogens.