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Infectious Diseases

Author(s): Christine Herrmann, PhD

Infectious Disease Agents

Most disease-causing organisms, or pathogens, are too small to be seen without a microscope. Some (e.g., most viruses) are even too small to be visible under a light microscope and must be viewed with the more powerful electron microscope. Because of their microscopic size, these minute organisms often are referred to as microbes or microorganisms. The study of these organisms is called microbiology, and scientists who study these organisms are microbiologists. Not all microbes cause disease; many are beneficial and even essential. Bacteria, in the digestive system, for example are important partners in digestion. Microbes that cause disease are sometimes informally referred to as “germs” or “bugs”.

The five main groups of pathogens are bacteria, viruses, protozoa, fungi, and helminths. Bacteria are simple, single-celled organisms that lack an organized nucleus or membrane enclosed organelles. They often have a cell wall (prokaryotes), and their cells usually are rod-shaped or spherical. Commonly known diseases caused by bacteria are diarrheal diseases, pneumonia, strep throat, tuberculosis, and anthrax. 

Viruses are particles of nucleic acid (DNA or RNA) surrounded by a protective coat that replicate within specific host cells and can spread from cell to cell. Infectious diseases caused by viruses include the flu, the common cold, AIDS, chickenpox, and hepatitis. 

Protozoa are single-celled, motile, eukaryotic organisms, found in the Kingdom Protista, that can be human parasites. A protozoan known as Plasmodium (over 170 species), causes malaria, an infectious disease that is one of the world’s top killers.

Fungi are made of eukaryotic cells (organized nucleus and membrane enclosed organelles). All fungi, with the exception of the yeast group, are multi-cellular organisms that absorb nutrients from the environment. Fungi can cause athlete’s foot, sinusitis, skin diseases, and vaginal infections.

Helminths (worms and flukes) are invertebrate animals, some of which are parasitic. Wuchereia bancrofti is transmitted to humans by way of the mosquito. The mature adults pass into lymphatic glands, obstructing lymphatic drainage and resulting in a disfiguring condition, known as elephantiasis.