An obligate intracellular parasite is a parasite that cannot reproduce outside of its host cell, relying entirely on the host cell's machinery to survive and replicate (e.g., viruses must infect cellular hosts to reproduce).

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Viruses are acellular microorganisms, meaning they are not composed of cells. A virus fundamentally consists of proteins and genetic material (either DNA or RNA, but never both) and remains inert outside of a host organism. To multiply and infect other hosts, viruses must incorporate themselves into a host cell and co-opt its cellular mechanisms. They can infect all types of cells, from human cells to other microorganisms, and while some are responsible for diseases like the common cold or Ebola, many do not cause disease.

Viruses

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

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- Animal viruses
- Bacteriophage 
- Plant viruses

Types of Viruses

There  is a wide range of viral structures such as
- Capsid
- Capsomer
- Viral envelope
- Spikes
- Sheath
- Tail fibers/pins
- Genetic Material

Common Viral Structures

Some common characteristics of viruses is that acellular, obligate intracellular parasites with host  specificity. They can have either a DNA or RNA genome. The genome can be surrounded by a protein capsid, or a phospholipid membrane for enveloped viruses. Many viruses lack the genes necessary for reproduction so they require host cell genomes for reproduction.

Characteristics of Viruses

Viral diseases encompass a wide range of infections and illnesses. Once a host cell is infected, the effects of the virus can vary, potentially causing cell death, abnormal growth, genomic alterations, or producing little to no noticeable impact. In humans, many viruses cause infections that are difficult to treat and eradicate due to a lack of effective medications.

Viral Disease

Unlike bacteria, many of which can grow on artificial nutrient media, viruses require a living host cell for replication. To isolate viruses, infected host cells are cultured in a liquid growth medium, which is then harvested. The virions can be separated from the host cells using centrifugation or filtration. In filtration, filters physically remove any components larger than the virions, allowing the viruses to be collected in the filtrate.

Isolation of Viruses

Due to their microscopic scale, viruses were not directly observed until the invention of the electron microscope in the late 1930s, although preventive treatments for viral diseases had been developed and implemented long before.

Visualization of Viruses

Obligate Intracellular Parasites

A virion is an acellular, fully infectious individual viral particle consisting of a nucleic acid genome—either DNA or RNA, but not both—surrounded by a protein capsid, with some also possessing a surrounding phospholipid membrane.

Virion

Viral titer is a measure of concentration defined as the number of virions present per unit volume of a given medium.

Viral Titer

Because viruses are obligate intracellular parasites, viral cultivation requires the presence of some form of host cell, which can be provided via a whole living organism (in vivo), a developing embryo, or cell cultures (in vitro).

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