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

Viruses, Viroids, Prions, and Viral Diseases

Definitions and Basic Concepts

Understanding the fundamental definitions and distinctions among viruses, viroids, prions, and bacteriophages is essential for microbiology students.

• Virus: An infectious agent composed of genetic material (DNA or RNA) surrounded by a protein coat (capsid), sometimes with a lipid envelope. Viruses require a host cell to replicate.

• Viroid: Small, circular RNA molecules without a protein coat, known to infect plants and cause disease.

• Prion: Infectious proteins lacking nucleic acids, responsible for neurodegenerative diseases such as Creutzfeldt-Jakob disease.

• Bacteriophage: Viruses that infect bacteria; structure includes a head (capsid), tail, and tail fibers. They play a key role in bacterial genetics and ecology.

Example: Prions cause mad cow disease, while viroids are responsible for potato spindle tuber disease.

Structure and Function of Viruses

Viruses exhibit diverse structures, which influence their infectivity and host range.

• Capsid: Protein shell that encases viral genetic material.

• Envelope: Lipid membrane derived from host cell, present in enveloped viruses; absent in naked viruses.

• Naked Virus: Lacks an envelope; more resistant to environmental stress.

• Enveloped Virus: Has an envelope; often more sensitive to desiccation and disinfectants.

Example: Influenza virus is enveloped, while adenovirus is naked.

Lytic and Lysogenic Cycles

Viruses replicate via two main cycles, especially bacteriophages.

• Lytic Cycle: Virus injects genetic material, hijacks host machinery, produces new virions, and lyses the cell.

• Lysogenic Cycle: Viral genome integrates into host DNA (prophage), replicates with host, may later enter lytic cycle.

Example: Lambda phage can undergo both cycles in Escherichia coli.

Key Equations:

• $\text{Viral Replication Rate} = \frac{\text{Number of Virions Produced}}{\text{Time}}$

Persistent Viral Infections

Some viruses cause persistent infections, remaining in the host for extended periods.

• Chronic Infection: Virus continuously replicates; e.g., Hepatitis B.

• Latent Infection: Virus remains dormant; e.g., Herpes simplex virus.

Properties of Seven Families of DNA Viruses

DNA viruses are classified into families based on structure, replication, and disease association.

Family	Diseases	Transmission	Diagnostics
Herpesviridae	Herpes simplex, Varicella (chickenpox)	Direct contact, respiratory	Serology, PCR
Poxviridae	Smallpox, Molluscum contagiosum	Direct contact	Clinical, PCR
Adenoviridae	Respiratory infections, conjunctivitis	Respiratory, fecal-oral	Culture, PCR
Papillomaviridae	Warts, cervical cancer	Direct contact	PCR, cytology
Hepadnaviridae	Hepatitis B	Blood, sexual	Serology, PCR
Parvoviridae	Fifth disease (erythema infectiosum)	Respiratory	Serology, PCR
Polyomaviridae	Progressive multifocal leukoencephalopathy	Respiratory	PCR

Additional info: Table entries inferred from standard DNA virus families.

Enteroviruses

Enteroviruses are RNA viruses in the Picornaviridae family, causing a range of diseases.

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

• Transmission: Fecal-oral route.

• Diagnostics: PCR, viral culture.

Viruses Causing Hepatitis (A, B, C, D, E)

Multiple viruses cause hepatitis, each with distinct properties.

Virus	Family	Transmission	Chronicity
Hepatitis A	Picornaviridae	Fecal-oral	No
Hepatitis B	Hepadnaviridae	Blood, sexual	Yes
Hepatitis C	Flaviviridae	Blood	Yes
Hepatitis D	Deltavirus	Blood, requires HBV	Yes
Hepatitis E	Hepeviridae	Fecal-oral	No

Additional info: Table entries inferred from standard hepatitis virus properties.

Diseases Caused by DNA and RNA Virus Families

Both DNA and RNA viruses are responsible for a wide range of human diseases.

• DNA Viruses: Herpesviruses (herpes, chickenpox), Poxviruses (smallpox), Papillomaviruses (warts).

• RNA Viruses: Influenza (flu), Rubella (German measles), Filoviruses (Ebola), Retroviruses (HIV).

Rubella

Rubella virus is an RNA virus causing German measles, characterized by rash and fever.

• Transmission: Respiratory droplets.

• Complications: Congenital rubella syndrome in pregnant women.

Poxviruses

Poxviruses are large DNA viruses causing diseases such as smallpox and molluscum contagiosum.

• Transmission: Direct contact, fomites.

• Diagnostics: Clinical presentation, PCR.

Herpesviruses

Herpesviruses are DNA viruses causing lifelong infections.

• Diseases: Herpes simplex (oral/genital), Varicella-zoster (chickenpox/shingles).

• Transmission: Direct contact, respiratory.

Filoviruses

Filoviruses are filamentous RNA viruses causing severe hemorrhagic fevers.

• Diseases: Ebola, Marburg.

• Transmission: Direct contact with body fluids.

Influenza

Influenza viruses are segmented RNA viruses causing seasonal flu epidemics.

• Transmission: Respiratory droplets.

• Diagnostics: Rapid antigen tests, PCR.

Hand, Foot, and Mouth Disease

Caused by enteroviruses, especially Coxsackievirus A.

• Symptoms: Fever, rash on hands, feet, and mouth.

• Transmission: Fecal-oral, respiratory.

Retroviruses: Oncogenic and Immunosuppressive

Retroviruses are RNA viruses that reverse transcribe their genome into DNA.

• Oncogenic Retroviruses: Cause cancer (e.g., Human T-cell leukemia virus).

• Immunosuppressive Retroviruses: Cause immune deficiency (e.g., HIV).

Key Equation:

• $\text{Reverse Transcription:} \, \text{RNA} \rightarrow \text{DNA}$

Additional info: Retroviruses use reverse transcriptase enzyme for replication.