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Unit 12: Viruses and Bacteria

Why a virus isn't alive, how bacteria multiply, and what makes the microbes living inside you your allies.

6 topics · 18 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

Why a Virus Isn't Considered Alive

  1. How it worksHow does a virus's genetic material hijack a host cell's own machinery to reproduce?
  2. Common mistakeWhy isn't a virus considered a living cell even though it carries genetic instructions?
  3. Going furtherWhat happens to a virus that never encounters a host cell to infect?
Your turn1 of 2
A virus particle sitting on a doorknob or countertop can remain completely inactive for weeks or even years, then suddenly begin replicating the moment it contacts a living host cell. Why can a virus sit inactive for so long and then suddenly spring into action?

Viral Structure and Replication

  1. How it worksWalk me through how a virus attaches to a host cell, injects its genes, and forces the cell to build more viruses.
  2. Going furtherWhat happens to a host cell once it bursts open, releasing hundreds of new viral copies?
  3. Common mistakeWhy doesn't a virus's protein shell count as a cell membrane?
Your turn1 of 2
A virus is built from little more than genetic material and a surrounding protein shell, a far simpler structure than even the simplest living cell. Why does a virus need a protein shell if all it really needs is its genetic material?

How Viruses Cause Disease

  1. How it worksHow does a virus destroying the host cells it infects trigger the symptoms of a viral illness?
  2. Common mistakeWhy doesn't taking an antibiotic help cure a viral infection like the flu?
  3. Ties togetherHow does your immune system's response to a virus sometimes cause more symptoms than the virus itself?
Your turn1 of 2
A cold virus doesn't directly consume your body's energy reserves or attack your muscles the way a physical injury might. Why does a cold virus still make you feel sick even though it never directly attacks your energy?

Bacterial Structure and Reproduction

  1. How it worksHow does binary fission split one bacterial cell into two identical daughter cells?
  2. Worked exampleFind how many bacteria result after 5 rounds of binary fission starting from a single cell.
  3. Common mistakeWhy isn't a bacterium the same thing as a virus even though both are microscopic?
Your turn1 of 2
Starting from a single bacterium, binary fission doubles the population with every round of division. After 16 rounds of division, how many bacteria are present?

Antibiotics and Antibiotic Resistance

  1. How it worksHow does natural selection favor the few bacteria that happen to survive an antibiotic dose?
  2. Common mistakeWhy doesn't an antibiotic cause a bacterium to develop resistance on purpose in response to the drug?
  3. Ties togetherHow does antibiotic resistance spreading through a bacterial population show natural selection happening in real time?
Your turn1 of 2
A doctor instructs a patient to finish an entire ten-day course of antibiotics, even if the patient starts feeling completely better after only three or four days. Why does finishing the entire course matter more than stopping once you feel better?

Beneficial Bacteria and the Human Microbiome

  1. How it worksHow do gut bacteria help break down food your own digestive enzymes can't fully process?
  2. Going furtherWhat happens to digestion and health when a course of antibiotics wipes out much of the gut microbiome?
  3. Common mistakeWhy isn't every bacterium in or on your body harmful just because bacteria can cause disease?
Your turn1 of 2
Your gut hosts trillions of bacteria, far outnumbering your own human cells, yet your immune system generally leaves them alone instead of attacking them the way it attacks harmful invading bacteria. Why doesn't your immune system attack these trillions of gut bacteria?