6: Acellular Pathogens

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6.1: Viruses
Viruses are generally ultramicroscopic, typically from 20 nm to 900 nm in length. Some large viruses have been found. Virions are acellular and consist of a nucleic acid, DNA or RNA, but not both, surrounded by a protein capsid. There may also be a phospholipid membrane surrounding the capsid. Viruses are obligate intracellular parasites.
6.2: The Viral Cycles
Many viruses target specific hosts or tissues. Some may have more than one host. Many viruses follow several stages to infect host cells. These stages include attachment, penetration, uncoating, biosynthesis, maturation, and release. Bacteriophages have a lytic or lysogenic cycle. The lytic cycle leads to the death of the host, whereas the lysogenic cycle leads to integration of phage into the host genome.
6.3: Isolation, Culture, and Identification of Viruses
Viral cultivation requires the presence of some form of host cell (whole organism, embryo, or cell culture). Viruses can be isolated from samples by filtration. Viral filtrate is a rich source of released virions. Bacteriophages are detected by presence of clear plaques on bacterial lawn. Animal and plant viruses are detected by cytopathic effects, molecular techniques (PCR, RT-PCR), enzyme immunoassays, and serological assays (hemagglutination assay, hemagglutination inhibition assay).
6.4: Viroids, Virusoids, and Prions
Other acellular agents such as viroids, virusoids, and prions also cause diseases. Viroids consist of small, naked ssRNAs that cause diseases in plants. Virusoids are ssRNAs that require other helper viruses to establish an infection. Prions are proteinaceous infectious particles that cause transmissible spongiform encephalopathies. Prions are extremely resistant to chemicals, heat, and radiation.
6.5: Viral Infections of the Skin and Eyes
Papillomas (warts) are caused by human papillomaviruses. Herpes simplex virus (especially HSV-1) mainly causes oral herpes, but lesions can appear on other areas of the skin and mucous membranes. Roseola and fifth disease are common viral illnesses that cause skin rashes; roseola is caused by HHV-6 and HHV-7 while fifth disease is caused by parvovirus 19. Viral conjunctivitis is often caused by adenoviruses and may be associated with the common cold. Herpes keratitis is caused by herpesviruses.
6.6: Viral Infections of the Respiratory Tract
Viruses cause respiratory tract infections more frequently than bacteria, and most viral infections lead to mild symptoms. The common cold can be caused by more than 200 viruses, typically rhinoviruses, coronaviruses, and adenoviruses, transmitted by direct contact, aerosols, or environmental surfaces. Due to its ability to rapidly mutate through antigenic drift and antigenic shift, influenza remains an important threat to human health. Two new influenza vaccines are developed annually.
6.7: Viral Infections of the Reproductive System
Genital herpes is usually caused by HSV-2 (although HSV-1 can also be responsible) and may cause the development of infectious, potentially recurrent vesicles. Neonatal herpes can occur in babies born to infected mothers and can cause symptoms that range from relatively mild (more common) to severe. Human papillomaviruses are the most common sexually transmitted viruses and include strains that cause genital warts as well as strains that cause cervical cancer.
6.8: Viral Infections of the Gastrointestinal Tract
Common viral causes of gastroenteritis include rotaviruses, noroviruses, and astroviruses. Hepatitis may be caused by several unrelated viruses: hepatitis viruses A, B, C, D, and E. The hepatitis viruses differ in their modes of transmission, treatment, and potential for chronic infection.
6.9: Viral Infections of the Circulatory and Lymphatic Systems
Viral pathogens of the circulatory system vary tremendously both in their virulence and distribution worldwide. Some of these pathogens are practically global in their distribution. Fortunately, the most ubiquitous viruses tend to produce the mildest forms of disease. In the majority of cases, those infected remain asymptomatic. On the other hand, other viruses are associated with life-threatening diseases that have impacted human history.
6.10: Acellular Pathogenic Diseases of the Nervous System
A number of different viruses and subviral particles can cause diseases that affect the nervous system. Viral diseases tend to be more common than bacterial infections of the nervous system today. Fortunately, viral infections are generally milder than their bacterial counterparts and often spontaneously resolve. Some of the more important acellular pathogens of the nervous system are described in this section.
6.11: Exercise

Thumbnail: This colorized transmission electron microscopic (TEM) image revealed some of the ultrastructural morphology displayed by an Ebola virus virion. (Public Domain; Frederick A. Murphy via CDC).