Infectious Disease: Evolving Challenges to Human Health

What Are Viruses, Bacteria, Fungi, and Parasites?

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Prions and Virods
The smallest known infectious agents, prions are composed of a single protein and viroids are a simple circle of ribonucleic acid (RNA). Prions infect brain tissue and cause normal proteins to change shape leading to cell death. Viroids infect plants and may affect the expression of genes.

Viruses are simple particles – as few as two genes inside protein shells – that must infect a host cell to grow and reproduce. Viruses use the host cells’ metabolic machinery to replicate themselves. Although viruses were once thought to be the smallest infectious organism, we now know that a single protein can reproduce itself and cause disease.

Bacteria are living, single-celled organisms. A bacteria cell has a cell membrane and a wall, but no nucleus. Some bacteria can survive harsh conditions as spores, then emerge and reproduce when conditions improve. Essential genes (DNA) can be found on a single or a handful of chromosomes; other genes may be found on smaller circular DNAs called plasmids.

Fungi have a wide array of shapes and complexity. Mushrooms are large fungi with many cell types. Yeasts and molds are single microscopic cells which can grow as colonies. Microscopic fungal cells have two types: round yeast cell or long thread-like cell chains. Fungi can reproduce by releasing spores, which can spread widely.

This classification covers a variety of organisms, from single-celled protozoa to much larger multi-cellular helminths (worms and flukes). They share many biological structures and metabolic processes in common with their plant and animal hosts. Genetic material (DNA) is contained on chromosomes, which are found in the nucleus of the cell. They contain numerous organelles within the cells, which carry out specific functions such as metabolism and food storage.

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Global Distribution of Disease [ next ]