Comprehensive Study Notes: Viruses, Viroids, Prions, and Viral Diseases

Viruses, Viroids, and Prions

Definitions and Key Concepts

Viruses, viroids, and prions are distinct infectious agents that differ in structure, genetic material, and mechanisms of infection. Understanding their definitions and properties is essential for microbiology students.

• Virus: An acellular infectious agent composed of nucleic acid (DNA or RNA) surrounded by a protein coat (capsid), and in some cases, a lipid envelope. Viruses require a host cell to replicate.

• Viroid: A small, circular, single-stranded RNA molecule that infects plants. Viroids lack a protein coat and do not encode proteins.

• Prion: An infectious protein that lacks nucleic acid. Prions cause neurodegenerative diseases by inducing abnormal folding of normal cellular proteins, especially in the brain.

• Bacteriophage: A virus that infects bacteria. Bacteriophages have complex structures, often with a head (capsid), tail, and tail fibers, and play a key role in bacterial genetics and ecology.

Example: The T4 bacteriophage infects Escherichia coli and is a model organism for studying viral replication cycles.

Viral Structure and Classification

Enveloped vs. Naked Viruses

Viruses are classified based on the presence or absence of a lipid envelope derived from the host cell membrane.

• Enveloped Virus: Possesses a lipid bilayer envelope surrounding the capsid. The envelope contains viral glycoproteins essential for host cell recognition and entry.

• Naked Virus: Lacks an envelope; only the nucleic acid and capsid are present. These viruses are generally more resistant to environmental stresses.

Example: Herpes simplex virus is enveloped, while Poliovirus is a naked virus.

Viral Replication Cycles

Lytic and Lysogenic Cycles

Bacteriophages and some animal viruses can undergo two main types of replication cycles:

• Lytic Cycle: The virus takes over the host cell machinery to replicate its genome and produce new virions, leading to cell lysis and release of progeny viruses.

• Lysogenic Cycle: The viral genome integrates into the host cell's chromosome (as a prophage) and replicates along with the host cell without causing immediate lysis. Environmental triggers can induce the prophage to enter the lytic cycle.

Example: Lambda phage can alternate between lytic and lysogenic cycles in E. coli.

Persistent Viral Infections

Viruses Causing Persistent Infections

Some viruses establish long-term infections in their hosts, either as chronic or latent infections.

• Chronic Infection: The virus is continuously present and often detectable (e.g., Hepatitis B virus).

• Latent Infection: The virus remains dormant within host cells and can reactivate later (e.g., Herpes simplex virus).

DNA Virus Families

Properties, Diseases, Transmission, and Diagnosis

The seven families of DNA viruses differ in structure, genome, and associated diseases. The following table summarizes key properties:

Family	Genome	Diseases	Transmission	Diagnosis
Adenoviridae	dsDNA, non-enveloped	Respiratory infections, conjunctivitis	Respiratory droplets, fecal-oral	PCR, antigen detection
Herpesviridae	dsDNA, enveloped	Herpes simplex, varicella-zoster, CMV	Direct contact, respiratory droplets	Serology, PCR
Poxviridae	dsDNA, enveloped	Smallpox, molluscum contagiosum	Direct contact, fomites	Electron microscopy, PCR
Hepadnaviridae	Partially dsDNA, enveloped	Hepatitis B	Blood, sexual, perinatal	Serology, PCR
Parvoviridae	ssDNA, non-enveloped	Erythema infectiosum (Fifth disease)	Respiratory droplets	Serology, PCR
Polyomaviridae	dsDNA, non-enveloped	Progressive multifocal leukoencephalopathy	Respiratory, urine	PCR
Papillomaviridae	dsDNA, non-enveloped	Warts, cervical cancer	Direct contact, sexual	PCR, cytology (Pap smear)

Additional info: Table entries inferred from standard virology sources.

RNA Viruses and Associated Diseases

Enteroviruses

Enteroviruses are small, non-enveloped, positive-sense ssRNA viruses in the family Picornaviridae. They include poliovirus, coxsackievirus, and echovirus.

• Diseases: Poliomyelitis, hand-foot-and-mouth disease, viral meningitis.

• Transmission: Fecal-oral route.

Viruses Causing Hepatitis

Several unrelated viruses cause hepatitis, classified as Hepatitis A, B, C, D, and E viruses.

Virus	Family	Genome	Transmission	Chronicity
Hepatitis A (HAV)	Picornaviridae	ssRNA (+)	Fecal-oral	No
Hepatitis B (HBV)	Hepadnaviridae	dsDNA	Blood, sexual, perinatal	Yes
Hepatitis C (HCV)	Flaviviridae	ssRNA (+)	Blood, sexual	Yes
Hepatitis D (HDV)	Deltavirus	ssRNA (-)	Blood, sexual	Yes (requires HBV)
Hepatitis E (HEV)	Hepeviridae	ssRNA (+)	Fecal-oral	No

Additional info: Table entries inferred from standard virology sources.

Diseases Caused by DNA and RNA Viruses

• DNA Viruses: Herpes simplex (cold sores, genital herpes), varicella-zoster (chickenpox, shingles), smallpox, warts (HPV).

• RNA Viruses: Influenza, measles, mumps, rubella, rabies, Ebola, hepatitis A/C/E, hand-foot-and-mouth disease.

Selected Viral Diseases

Rubella

Rubella (German measles) is caused by the Rubella virus (Togaviridae, ssRNA). It is transmitted via respiratory droplets and can cause congenital rubella syndrome if infection occurs during pregnancy.

Poxviruses

Poxviruses (Poxviridae) are large, enveloped, dsDNA viruses. They cause diseases such as smallpox and molluscum contagiosum. Transmission is via direct contact or fomites.

Herpesviruses

Herpesviruses (Herpesviridae) are enveloped, dsDNA viruses. They cause oral and genital herpes, chickenpox, shingles, cytomegalovirus infection, and more. Transmission is typically via direct contact or respiratory droplets.

Filoviruses

Filoviruses (Filoviridae) are filamentous, enveloped, negative-sense ssRNA viruses. They include Ebola and Marburg viruses, which cause severe hemorrhagic fevers. Transmission is via contact with bodily fluids.

Influenza

Influenza viruses (Orthomyxoviridae) are enveloped, segmented, negative-sense ssRNA viruses. They cause seasonal flu and pandemics. Transmission is via respiratory droplets.

Hand, Foot, and Mouth Disease

This disease is most commonly caused by Coxsackievirus A16 (an enterovirus). It presents with fever, mouth sores, and a rash on hands and feet. Transmission is fecal-oral or via respiratory droplets.

Retroviruses

Oncogenic and Immunosuppressive Retroviruses

Retroviruses are enveloped, positive-sense ssRNA viruses that replicate via a DNA intermediate using reverse transcriptase.

• Oncogenic Retroviruses: Can induce cancer by integrating into host DNA and activating oncogenes (e.g., Human T-cell leukemia virus, HTLV).

• Immunosuppressive Retroviruses: Cause immune system dysfunction (e.g., Human Immunodeficiency Virus, HIV, which leads to AIDS).

Example: HIV infects CD4+ T cells, leading to progressive immunodeficiency.