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Pathogens and Disease

Viruses: Introduction

A virus can be simply defined as an obligate intracellular parasite. Each viral particle, or virion, consists of a single nucleic acid, RNA or DNA, encoding the viral genome surrounded by a protein coat, and is capable of replication only within the living cells of bacteria, animals or plants. Viruses are classified into different orders and families by consideration of the type of nucleic acid present (RNA or DNA), whether the nucleic acid is single- or double-stranded, and the presence or absence of an envelope.

Virus replication

As viruses are obligate intracellular pathogens they cannot replicate without the machinery and metabolism of a host cell. Although the replicative life cycle of viruses differs greatly between species and category of virus, there are six basic stages that are essential for viral replication.

1. Attachment: Viral proteins on the capsid or phospholipid envelope interact with specific receptors on the host cellular surface. This specificity determines the host range (tropism) of a virus.


Yersinia enterocolitica is the etiological agent of a gastroenteritis, known as yersiniosis. Occasionally this can give rise to a pseudoappendicitis, which is more severe than that caused by Y.pseudotuberculosis and can involve an acute terminal ileitis and mesenteric lymphadenitis. Yersiniosis often results in an immune disregulation which is manifest as a reactive arthritis. Unlike the other pathogenic Yersinia, Y.enterocolitica is not thought to be a true zoonosis, being transmitted by the ingestion of infected animal products or by contact.