Introduction to Virus Classes and Families

Viral Classification Systems and the Six Classes of Viruses

• Viruses do not fit into the standard biological classification domains (Domains, Kingdoms, etc.); they have a unique classification system.

• The system divides viruses into classes based on their genetic material and replication method.

• Although logical divisions suggest four basic types, there are actually six official classes discussed:     - Class 1: Double-stranded DNA ($dsDNA$).     - Class 2: Single-stranded DNA ($ssDNA$).     - Class 3: Double-stranded RNA ($dsRNA$).     - Class 4: Single-stranded RNA ($ssRNA$) where the RNA acts directly as mRNA.     - Class 5: Single-stranded RNA ($ssRNA$) where the RNA is the complement used to create mRNA.     - Class 6: Single-stranded RNA ($ssRNA$) that serves as a template for DNA synthesis (Retroviruses).

• The technical reasons for the distinction between Classes 4, 5, and 6 involve how the RNA is utilized:     - In Class 4, the viral RNA is essentially the mRNA.     - In Class 5, the viral RNA is a template to make the actual mRNA using base pairing ($A$ goes with $U$ and $C$ goes with $G$).     - In Class 6 (Retroviruses), the virus uses RNA to make DNA first, then uses that DNA to produce new viral particles.

• An "envelope" refers to a membrane surrounding the virus; the presence of this is often noted in classification tables.

Class 1: Double-Stranded DNA ($dsDNA$) Virus Families

• Adenovirus Family     - These viruses are non-enveloped and commonly cause respiratory diseases.     - Most humans have likely experienced an adenovirus infection, often mistaking it for a common cold.     - While most are not severe, some adenoviruses are linked to the development of tumors (cancer).

• Papovavirus Family     - Human Papillomavirus (HPV): The most well-known human virus in this family. It is a sexually transmitted infection ($STI$).     - Symptoms: Causes warts, which are bumpy skin growths that can appear on the genitals.     - Vaccination: A very effective vaccine exists, recommended for children around age $9$ to ensure coverage before sexual activity begins.     - Policy Debate: In Michigan, there was a controversy regarding adding the HPV vaccine to required school immunizations for students in approximately $4th$ or $5th$ grade. Opponents argued it was unnecessary for children, and the requirement did not pass in an election.     - Cancer Connection: HPV can cause cervical cancer in women. Men do not have a cervix and thus do not get cervical cancer, but they can carry and transmit the virus.     - Risk: Only a fraction of a percent of infected women develop cancer, and it can take $15$ to $25$ years to manifest after infection.

• Herpesvirus Family     - Includes various strains that integrate their DNA into the host's DNA, making the infection permanent (no cure).     - Herpes Simplex 1 (HSV-1): Causes cold sores near the lips.     - Herpes Simplex 2 (HSV-2): Causes genital sores; it is sexually transmitted.     - Latency: The virus remains dormant (inactive) in cells between outbreaks. A person may appear cured when sores vanish, but the virus is still present and can reactivate weeks, months, or years later.     - Testing Limitations: Many strains of sexually transmitted herpes do not cause illness, and there are currently no commercially available tests for these asymptomatic strains. Finding them would require personal DNA sequencing.     - Cost of Sequencing: The first human genome sequence cost approximately $2,000,000,000$. Modern technology has reduced this to about $30,000$, though this is still too expensive for routine testing.     - Varicella Zoster Virus: Causes chickenpox and shingles. Approximately $95\%$ of people have been exposed or infected. It also integrates into host DNA permanently.     - Epstein-Barr Virus: Named after scientists Epstein and Barr. It primarily causes mononucleosis (commonly called "mono"), which can be sexually transmitted. In a small percentage of cases, it can lead to Burkitt's lymphoma (a type of blood cancer).

Viral Oncogenesis: How Viruses Cause Cancer

• Viruses that integrate their DNA into the host genome must "cut" the host's DNA to "glue" the viral DNA inside.

• The human genome contains genes that regulate the cell cycle.

• Mechanism: If a virus accidentally cuts and inserts itself into a gene controlling the cell cycle, that gene is disabled. Consequently, the cell loses control and begins dividing uncontrollably, forming tumors/cancer.

• Probability: Because the human genome is very long and cell-cycle genes are relatively small and specific, this event is statistically unlikely, explaining why only a small percentage of infected individuals develop cancer.

The Poxvirus Family and the History of Vaccination

• Smallpox Virus: A historically devastating disease characterized by painful pustules (blisters) across the entire body, leading to death or severe disfigurement/scarring.

• Eradication: The United Nations led a global vaccination campaign in the $1960s$ and $1970s$. By vaccinating approximately $95\%$ to $98\%$ of the world, the virus was eliminated from nature.

• Current Status: Smallpox only exists in two known locations: the US Army and the Russian Army labs, held potentially as biological weapons. Vaccinations are no longer given to the general public, though soldiers might still receive them.

• Origins of Vaccines (The Cowpox Connection): In the Middle Ages ($1600s$-$1700s$), it was noticed that milkmaids (women milking cows) were immune to smallpox. They were actually catching Cowpox, a mild related virus.

• First Immunization: Parents would purposely infect children with cowpox scabs. The antibodies created against the mild cowpox virus provided cross-protection against the lethal smallpox virus.

Class 2 and Class 3 Viruses

• Class 2: Single-stranded DNA ($ssDNA$)     - These viruses lack a double helix structure.     - Arbovirus Family: Includes strains like B19 parvovirus.     - Symptoms: Typically causes a mild rash (red, itchy dots) that usually clears on its own with rest and hydration.

• Class 3: Double-stranded RNA ($dsRNA$)     - Unusual because RNA is typically single-stranded.     - Reovirus Family: Includes the Rotavirus, a frequent cause of diarrhea.     - Physiology of Diarrhea: The large intestine is responsible for water absorption. Irritation from viruses, bacteria, or protozoa prevents this function, leaving water in the intestine.     - Colorado Tick Fever Virus: Transmitted by ticks (insects). Found in Colorado and other regions.     - Tick Transmission: Ticks are arachnids/insects that crawl for hours seeking warm, hidden areas (behind ears, groin, armpits, behind knees).     - Prevention: Checking for ticks after hiking is essential. To remove a biting tick, use tweezers to grab the head close to the skin and pull without squishing the body, as squishing can force infected fluids into the host.     - Lyme Disease: Mentioned as another tick-borne illness (caused by bacteria/protozoa, not a virus).

Class 4: Single-stranded RNA ($ssRNA$) Viruses

• Picornavirus Family     - Rhinovirus: The cause of the common cold.     - Poliovirus: Causes Polio, which infects nerves and leads to paralysis. If the nerve for the diaphragm is affected, the patient cannot breathe.     - Historical Treatment: Patients were placed in "iron lungs" (pressure chambers) to force breathing. Modern medicine uses respirators.     - Recovery: Some recover fully; others recover only $60\%$ to $80\%$ and require canes or wheelchairs.     - Notable Figures:         - Franklin D. Roosevelt (FDR): Used a wheelchair due to a childhood infection (the transcript mentions COVID, though polio is implied by the context of a wheelchair and historical timing).         - Mitch McConnell: Republican Senator who had polio as a child before the vaccine was available. His personal experience with the severity of the disease and the efficacy of vaccines likely influenced his quiet stance during COVID-19 vaccine skepticism in his party.

• Coronavirus Family     - Includes COVID-19 (the pandemic strain) and earlier outbreaks of SARS (Severe Acute Respiratory Syndrome).     - SARS-1: Occurred in China approximately $20$ to $30$ years ago; was deadly but did not spread globally.     - MERS/Middle East Outbreaks: Another coronavirus outbreak occurred in Saudi Arabia around the same time.     - Communication Strategy: Officials named the pandemic "COVID-19" rather than "SARS-2" to avoid international panic, as people in Asia/Africa were already familiar with the high mortality of SARS.     - Scientific Adaptation: Early pandemic guidance (e.g., masks) changed as scientists realized COVID-19 behaved slightly differently than other family members they had studied previously.

Class 5: Single-stranded RNA ($ssRNA$) Viruses and Others

• Flavivirus Family     - Includes Yellow Fever, West Nile Virus, and Hepatitis C.     - West Nile Virus: Native to the Nile region in Africa but spread across North America within $3$ years. Transmitted by mosquitoes. Highly lethal to birds (evidenced by "three-dimensional" dead birds on streets, not flattened by cars). In Anaheim, Costa Mesa, and Santa Ana, cases reached numbers like $7$, $14$, and $10$ during outbreaks.

• Togavirus Family     - Includes Rubella, part of the MMR Vaccine (Measles, Mumps, Rubella).

• Filovirus (The Ebola Virus)     - Causes Hemorrhagic Fever. It reproduces in internal organ tissues, causing them to break apart.     - Symptoms: Internal bleeding followed by bleeding from every body opening—anus, mouth, nose, ears, eyes (tear ducts), urethra, and vagina.     - Transmission: Not airborne; transmitted through bodily fluids (blood).     - Prevention: Use heavy-duty PPE (Personal Protective Equipment) including sealed suits and internal airbags for breathing (specifically used by doctors/nurses).     - Mortality and Treatment: The latest outbreak had a $30\%$ mortality rate. A biotech company in San Diego developed a medicine/vaccine that was tested for the first time during a recent African outbreak, showing promise, though further testing is needed.

Questions & Discussion

• Student Question: Why don't men get cervical cancer?

• Speaker Response: Men do not have a cervix as part of their reproductive system. However, they can carry and transmit the HPV virus to women.

• Discussion on Ticks: The speaker noted a country song titled "I Want to Check You for Ticks" as a reminder for post-hiking checks.

• Comparison of Fears: During a California West Nile outbreak, people were more afraid of Ebola due to news coverage, despite West Nile being the more immediate local threat.