I forgot the most important part! The definitions/descriptions I used
I say descriptions because I really wanted to get across the point to Alex that although we tend to think of germs as nasty little things to avoid at all costs, there is much more too it! When she first learned I taught her more about 'helpful' bacteria than 'harmful'...as should be the case!
So...a bit more info, simplified for toddlers. You may want to add more if you have given them previous background.
1. Bacteria- single-celled microorganism (living thing) that can live by itself or with other bacteria in colonies. They are found literally everywhere: dirt, air, water, in animals, plants, and people. [This is a really fun time to introduce the word 'ubiquitous' as I have found that many children LOVE the way it sounds
Alex would say it then run around madly waving arms shouting 'the're EVERYWHERE!! - she still loves the word and uses it every chance she gets, which solidifies the concept:)]
Whilst many bacteria do indeed cause disease and illness, most are beneficial: some examples of this are found in the food industry, like in making cheese, yogurt, pickles. Other examples can be found in our digestive track as they assist in breaking down fiber from plants. Still others do amazing things in nature and for the environment, such as helping to clean up oil spills!
WARNING: In your discussion of indigestible plant fiber, you may in fact be tempted to use corn as an example...whilst this is quite logical, you just might be setting your child up with a slightly embarrassing fascination that you, as the parent, will deal with. In public restrooms. Loudly. My child wanted to ask perfect strangers about its presence or lack thereof....
On the other hand, if I ever have a boy, I shall take great joy in repeating this lesson so that his father can share in the experience that he found so comical
2. Viruses- ultra microscopic 'agent' that is infectious, and can only replicate (make copies of itself) inside the the cells of a living thing. Many cause disease, but there are some beneficial effects of viruses. For example, they can be used to make vaccines, and some (bacteriophages) kill harmful bacteria...there have even been reported cases of individuals being cured of a bacterial disease when exposed to viruses at the same time.
3. Fungi- this one gets a bit more difficult because it is such a broad grouping uses new language to most kids.. But they are all Eukaryotes, reproduce by spores, and live off of other organisms (parasitic). they also dont have leaves or chlorophyll. Mushrooms, lichens, penicillin, bread mold are all types of common fungi, so this leads to a nice conversation about benefits.
4. Protozoa- there actually isn't a proper definition here as they are a problem area for taxonomists, and are usually umbrella'd under Protists...but they are usually microscopic, single-celled, and are either parasitic or symbiotic. Some are human parasites like amoebas, Malaria, Giardiasis, etc...
They can be beneficial as well by improving the soil or water as they eat other microorganisms. Alex found it funny that termites can only digest wood with the help of Protozoa in its digestive track. Diatoms are another good example...
We did use this as a good example of why we shouldn't drink from any random sources of water outside...in ponds. Puddles. Buckets of suspect water. Sigh.
Ah, well. Bit of a long post!
Hope it is remotely useful