Week 2: Activity One: What is a Virus?Pathogens Cause Infectious Disease

A disease is any condition of the body that causes pain, illness, or death. Non-infectious diseases, such as genetic diseases and cancer, cannot spread from person to person. Other diseases are infectious, which means that they can spread from one person to another.

Pathogens, which are particles or organisms that infect sick individuals, cause infectious diseases. Pathogens destroy healthy cells in order to survive and reproduce. But reproducing in the body of their current host will only take a pathogen so far. In order to continue their lifestyle, they must spread to a new host. There are six main types of pathogens. Examine the six types listed on the next page, and classify them by placing them into the boxes below. Just to make sure that you are up on your game, define “prokaryote” and “eukaryote” below. If you can’t remember, use your textbook or phone to look them up.

Prokaryote: _________________________________________________________Eukaryote: __________________________________________________________

Pathogens can be…Non-living particles1. ___________________2. __________________

Prokaryotes1. ___________________

Eukaryotes 1. ___________________2. ___________________3. ___________________

People often mix up bacteria and viruses, but they are tremendously different. For one thing, as you wrote above, bacteria are single celled organisms without a nucleus- but viruses aren’t even alive. Another major difference between viruses and bacteria is size. If a red blood cell were about the size of this sheet of paper, then a bacterium and some virus particles would be about the size shown to the left.

Pathogens Cause Infectious DiseaseType

of Pathog

en

Example Description Human diseases caused by pathogens

of that type

Prions Malformed proteins that cause other proteins to fold incorrectly

Bovine spongiform encephalopathy (Mad cow disease), Creutzfeldt–Jakob disease (CJD), Kuru

Virus Coronavirus Non-living particles that reproduce by taking over living cells

Common cold, flu, cold sores, measles, chicken pox, small pox

Bacteria

Escherichia coli Single celled prokaryotes

Strep throat, staph infections, tuberculosis, food poisoning, tetanus

Fungus Candida albicans Organisms, including mushrooms and yeasts, that grow as single cells or thread-like filaments.

Ringworm, athlete’s foot, candidiasis, histoplasmosis

Protozoa (NOTE: protozoa can be classifi

Trypanosoma brucei Single celled eukaryotes that do not fit into the eukaryotic categories of animals, plants, or fungus.

Malaria, “traveller’s diarrhea”, giardiasis, trypanosomiasis (“sleeping sickness”), Toxoplasma gondii

ed as parasites)Parasites

Taenia solium Multicellular organisms that survive by harming a host

Tapeworm, guinea worm, hookworm, blood flukes, head lice, ticks

On the first page, if you put “virus” into the category of non-living particle, you are correct! But how can a virus infect us, reproduce, and make us sick if it isn’t even alive? To determine whether a virus is alive or not, we could compare the virus's characteristics to what many biologists consider the requirements of life. All living things have several common characteristics. Some nonliving things may have one or more of the characteristics but not all of them. If a virus were to be classified as alive, then it must do ALL of the following things, which it does not. Have Movement Perform Respiration Have Sensitivity

Grow, develop, and die Be Organized Have Nutrition (it

needs to eat something)

Have Excretion (it needs to get rid of wastes)

Reproduce Evolve over time

Response Questions1. Put red blood cells, viruses, and bacteria in size order, from smallest to largest.

____________________ ___________________ _____________________Smallest-----------------------------------------------------------Largest

2. If a red blood cell were the size of your car, how big would a bacteria be? _______________________ How big would a coronavirus be? __________________________________________

Consider the paragraph at the top of this page. 3. Which of the life functions listed do viruses perform? ________________________

____________________________________________________________________________________________________________________________________4. Which of the life functions listed do viruses NOT perform? ____________________ ____________________________________________________________________________________________________________________________________5. Scientists do not have a clear sense of when viruses evolved, as they leave no fossils, and mutate so quickly. Some scientists think that viruses were one of the first things to evolve and may represent a link between non-living things and living things. Other scientists think that viruses may be descendants of life forms that were once alive but degenerated over time. Based on what you have studied today, which of these do you find to be more plausible? Why? ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________