A disease is the abnormal functioning of
the body that prevents the body organs or processes from functioning as they
should. There are many causes of disease. Disease can be caused by fungi (i.e.
athlete's foot) bacteria (i.e. tuberculosis), virus (i.e. common cold) and
protozoa (amoebic dysentery). Not all types of fungi, bacteria, viruses and
protozoa and disease-causing agents. The causes of disease are sometimes
referred to as germs. The definition of the term germ is sometimes ambiguous.
Germs can be carried through food, water, air, animals, and people.
"Bad" germs are pathogenic or disease causing organisms and are the
ones that make people sick.
Bacteria, viruses, and protozoa cause different infections. The
worksheet is just a way for student to look at these "germs" without
a microscope. They should look like the following:
Bacteria are extremely small
organisms. They are so small that you cannot see them with your naked eye, and
need at least 400 times magnification with your microscope to begin seeing
them. Identifying bacteria is very difficult since they are very small. Many
times scientists have to make a culture of a suspect bacteria to find out what
problem the suspect bacteria may have caused. This culture is allowed to grow
in a petri dish and the colony of bacteria is then observed. Bacterial
diseases include pneumonia, strep throat, trench mouth, boils, blood
poisoning, gonorrhea, chancre sores, bubonic plague, cholera, and syphilis.
Viruses are composed of a protein overcoat
enclosing a core of either DNA or RNA. Once a virus is attached to a
susceptible cell it dissolves a hole through the membrane and uses the cell's
processes to reproduce. Diseases that are caused by viruses include the common
cold, influenza, herpes, yellow fever, shingles, rabies, polio, and
Protozoa are single celled organisms. Most
protozoa do not cause infections, but a few do. Malaria, dysentery, African
sleeping sickness are caused by different species of protozoa.
- You may want to prepare agar solutions and grow
bacteria cultures to show your students. We do not recommend that the students
do this, the germs could get out of control! It is necessary to stress
that probably all of the bacteria grown will be harmless, but that the
students shouldn't touch the cultures with their hands. Make sure
that they wash their hands if they touch the cultures
- Below are materials and instructions that can be
used to make your own cultures. You can also purchase ready-made
agar petri dishes. You can also use a thin layer of pectin (usually in
the grocery store with “Canning” supplies.)
MATERIALS: agar, water, glassware, hot plates, petri dishes, tape,
- Make sure that all the glass and petri dishes
that you use are clean. If not, wash with soap and water
and let stand to dry upside down. Do not dry with a cloth or
- Weigh about 2.5 grams of agar from the bottle
labeled NUTRIENT AGAR.
- Fill a beaker with 100 ml of water and add agar
- Boil the solution and be sure to stir
frequently. Keep your eyes and face away from the top of the
- Let the solution stand at 100 degrees centigrade
for 15-20 minutes.
- Pour the solutions into the clean petri dishes
and tape the dish around the edges where the two tops meet.
- Label the dishes and store them until the next
**NOTE: THE INSTRUCTIONS ABOVE MAKE ENOUGH MATERIAL FOR ONE
PERSON. YOU WILL HAVE TO ADJUST THE AMOUNT OF AGAR AND WATER DEPENDING
ON YOUR CLASS SIZE.
- You can then prepare your bacteria colonies by inoculating
the agar. Remove the tape from the petri dish and take the top cover
off. With a clean toothpick make two lines so that there are 4 sections
in the dish. With a clean toothpick scrape the inside of your mouth.
Do not poke your skin, just scrape your inside of the cheek. Rub
the toothpick on the agar, very gently. DO NOT poke or scratch the
agar, just rub it. On the other section, place a piece of hair, spit,
scab, or anything else to see if bacteria will grow.
- After you have inoculated all 4 sections tape the petri
dish. Store in a place that is warm (not hot). After you see
growth, we recommend that you use a Microbiology book if you want to identify
your colony of bacteria. Discard the germs with care under sanitary
- Use the worksheet and have the students create a picture
of each of the different types of diseases. You may also want to
do an Internet search to find out more information on diseases.