CL

Attributes of Living Things: Diseases, Viruses, and Evolution

  • Diseases: Broad Classification

    • A disease is anything that makes you sick.
    • Infectious Diseases:
    • Caught and spread rapidly from person to person.
    • Acquired from the environment or another person.
    • Can spread directly (e.g., sneezing in someone's face giving them the flu) or indirectly (e.g., sneezing on a chair, and someone else later contacts the viruses).
    • Genetic Diseases:
    • Diseases of DNA, something an individual is born with, not generally 'caught'.
    • Examples: Spina bifida, hemophilia.
    • Can manifest at different times in life, potentially due to DNA damage.
    • Will be discussed in the third part of the class, after October 14.

  • Science and Vaccines

    • Science must be testable, repeatable, and falsifiable.
    • The method of curbing infectious disease spread, such as through vaccines, aligns with the scientific method:
    • Testable: Vaccines are tested by administering them to one group and comparing sickness rates to a control group.
    • Repeatable: Multiple trials can be conducted.
    • Falsifiable: A vaccine either protects against a virus or it doesn't.
    • Most college students are vaccinated for various diseases as a condition of enrollment.

  • Measles Epidemic and Complications

    • A recent case in LA County involved a child who died from rare complications of measles, contracted as an infant before being eligible for the vaccine (recommended age: 12 to 15 months).
    • The United States is experiencing a measles epidemic:
    • As of September 16, there were 1491 cases of measles in the U.S. (compared to circa 240 last year).
    • 92\% of these individuals were unvaccinated.
    • 4\% had received one dose, and another 4\% had received two doses.
    • Infection rates are significantly higher among unvaccinated populations.
    • Herd Immunity: The unvaccinated infant likely contracted measles from an unvaccinated person, highlighting how widespread vaccination protects vulnerable individuals who cannot receive vaccines (e.g., newborns).
    • Impact of Lockdowns: A decrease in measles transmission (and other infectious diseases like common cold, flu, COVID) was observed in 2020 due to lockdowns and reduced social mingling.
    • Transmission Rates: The average person with flu infects 1.4 people; a person with measles infects 12 to 18 people (an order of magnitude higher).
    • Measles was declared eliminated in the U.S. in 2000 but has re-emerged due to decreased vaccination rates, which can be linked to institutional distrust.

  • MMR Vaccine and Autism Controversy

    • Origin of the Link: In the 1990s, a British doctor named Andrew Wakefield published a paper in February 1998 attempting to link the MMR vaccine to autism.
    • Wakefield's Study: Titled "Ileal Lymphoid Nodular Hyperplasia, Non-Specific Colitis, and Pervasive Developmental Disorder in Children," it involved 12 children. 8 of the 12 children reportedly developed behavioral symptoms (autism spectrum disorder) after receiving the vaccine.
    • Fraudulent Findings and Retraction:
    • Wakefield was found to have been paid approximately \pounds400,000 (circa 660,000 USD) by a law firm preparing a legal case against a vaccine manufacturer.
    • More than half the children in the study were plaintiffs in this legal case.
    • Wakefield had also filed a patent for an alternative measles vaccine shortly before publishing the paper, creating a clear conflict of interest.
    • The paper was retracted in 2010, 12 years later, after being deemed fraudulent.
    • Subsequent large-scale studies, like a Danish study involving \sim2,000 children, found no link between MMR vaccines and autism.

  • Tylenol and Autism Controversy

    • A recent study suggested an association between prenatal Tylenol (acetaminophen) exposure and an increased risk of ADHD and autism spectrum disorder (ASD) in children, examining umbilical cord blood biomarkers in \sim996 individuals.
    • Association vs. Causation: The crucial distinction is between association and cause-and-effect. An example given is that everyone who drank a glass of water in 1890 is dead, but this doesn't mean drinking water caused their deaths.
    • Sibling Control Study: A large-scale study in Sweden (2.48 million children over 25 years) used a sibling control design (comparing children from the same mother, one exposed to acetaminophen prenatally, one not). This study found no correlation, suggesting earlier associations might be due to confounding factors (e.g., other reasons why a pregnant mother might take Tylenol).
    • Some infections, not just genetic factors, can also lead to autism.

  • Infectious Diseases: Fundamental Questions

    • What constitutes an infection or an infectious disease?
    • Why are there so many agents that cause disease (pathogens)?
    • Why do individuals need repeated vaccinations (e.g., annual flu shots, multiple COVID boosters)?

  • Pathogens

    • Definition: An agent that infects or invades tissues and causes sickness.
    • Types:
    • Bacteria (singular: bacterium): Microorganisms like Salmonella, Anthrax, Staphylococcus (staph infection), Borrelia (causes Lyme disease).
    • Viruses: Like influenza virus, HIV, coronavirus, measles virus.
    • Other Microorganisms: E.g., Naegleria (brain-eating amoeba, found in Santee Lakes, high mortality), Plasmodium (causes malaria).

  • Bacteria: Harmful and Beneficial Roles

    • Not all bacteria are harmful; some are essential.
    • Harmful Examples: Salmonella (food poisoning), Borrelia (Lyme disease from tick bites).
    • Beneficial Role: Nitrogen Fixation: Bacteria pull nitrogen from the atmosphere and convert it into a form plants can use for DNA synthesis. Humans then obtain nitrogen by eating plants, forming a mutual symbiotic relationship in the nitrogen cycle.
    • Abundance: There are more individual bacteria on Earth than all other organisms combined, primarily due to their small size, though there are more species of plants than bacteria, and bacteria do not have the highest biomass.
    • Many more bacteria live inside human intestines than there are people on Earth, demonstrating their ubiquity.

  • Viruses: Nature and Mechanism

    • Controversy of Living Status: Viruses do not meet all seven fundamental characteristics of life.
    • They have DNA (or RNA) (genetic material), enclosed in a protein capsid.
    • They evolve.
    • However, they do not grow, develop, have cells, or reproduce on their own.
    • Therefore, they are technically not considered living things by some definitions.
    • Examples: Influenza, measles, HIV, coronaviruses (SARS-CoV-2), tobacco mosaic virus (the first discovered virus, affecting plants).
    • Structure: Primarily composed of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) surrounded by a protein coat called a capsid.
    • Recognition Spikes: Proteins on the surface of the capsid act like 'keys' that allow viruses to trick host cells into letting them in.
    • Bacteriophage: A specific type of virus that attacks bacteria. Its name means "bacteria-eating virus."
    • Infection Mechanism: The Lytic Cycle:
    • A virus injects its DNA/RNA into a host cell (e.g., a bacterium).
    • The host cell mistakes the viral genetic material for its own and starts reading it.
    • The viral genetic material contains instructions for making new viruses.
    • The host cell's machinery is hijacked to produce many copies of the virus.
    • New viruses assemble inside the cell.
    • The host cell lyses (ruptures/breaks), releasing numerous new viruses that go on to infect other cells.
    • This rapid replication (1 virus \to 10 viruses in 20 minutes in bacteria) explains quick spread.
    • Prophage (Lysogenic Cycle in bacteria):
    • Instead of immediately replicating, viral DNA can integrate into the host cell's (e.g., bacterial) DNA (genome).
    • The host cell continues its normal life and divides, copying its own DNA along with the integrated viral DNA (prophage).
    • This creates multiple daughter cells, all carrying the viral DNA.
    • Under certain conditions, the prophage can pop out of the host DNA, initiating the lytic cycle in all those cells, leading to a massive release of viruses simultaneously.
    • Human Infection: Similar processes occur in human cells; a single virus infects a cell, replicates, the cell bursts (unnoticed individually), and dozens of new viruses infect other cells, leading to a rapid spread throughout the body and causing sickness.
    • Viruses can also be shed from the body (e.g., nose, mouth) to infect other people.

  • Zoonotic Diseases and Emerging Viruses

    • Zoonosis/Zoonotic Diseases: The spread of pathogens that jump from animals (wild or domestic) to humans, and vice versa (e.g., humans transmitting COVID-19 to pets).
    • Vectors: Animals (e.g., rodents, mosquitoes, fleas, ticks) that transport pathogens from one individual or species to another.
    • Examples: Fleas on rodents carrying Plasmodium (malaria), mosquitoes carrying West Nile and Zika viruses.
    • Vector control (e.g., mosquito spraying in San Diego) is crucial for public health.
    • Climate Change Impact: Rising temperatures lead to longer summers and shorter winters, expanding the ranges and exposure times to vectors like mosquitoes, increasing the likelihood of pathogen transmission and global pandemics.
    • Emerging Viruses: Viruses previously unknown in humans that suddenly appear and spread rapidly within the population.
    • HIV: Jumped from primates (SIV) to humans in the 1980s, causing a major pandemic.
    • Avian Flu and Swine Flu: Many flu viruses originate from birds (poultry) and integrate into human populations.
    • Ebola Virus and Zika Virus (2016-2017).
    • Hepatitis A Virus (2017): Local outbreak in San Diego, affecting the homeless population.
    • SARS-CoV-2 (2019):
      • Virus name: Severe Acute Respiratory Syndrome Coronavirus Number Two.
      • Disease name: COVID-19 (Coronavirus Disease 2019).
      • Thought to have originated from a wild animal (potentially involving a raccoon dog as an intermediate host) and jumped to humans.
      • SARS-CoV-1 (the first SARS) caused a deadly outbreak in Asia in 2002-2003 but was eradicated. SARS-CoV-2 has not been eradicated.

  • The "One Word Answer": Evolution

    • Why So Many Pathogens? Why Annual Vaccines? The answer is evolution.
    • Pathogens are products of evolution; they constantly change, making it difficult to achieve permanent immunity or eradication with a single vaccine.
    • Charles Darwin's Definition: Descent with modification. Individuals are descended from parents but are modified (look different).
    • Modern Definition of Evolution: Genetic change in a population or a species over generations.
    • Relies on the understanding of DNA, genes, and genetics (knowledge Darwin lacked).
    • If the genetic composition of a population changes from one generation to the next, that population has evolved.
    • Evolution explains the diversity of life, including beneficial organisms, but also the persistence and challenge of pathogens.