Bacteria are the little creatures that are too small to see with a microscope. They are responsible for a number of illnesses. They help you digest your food. They outnumber all of humanity. They are everywhere, in our bodies, in our neighbor’s bodies, in our water, in our food, and on our hands. Most people think that bacteria are causes of disease and that we would be better off if they didn't exist. Not all of them are bad though, but that doesn't mean you shouldn't be careful.
Bacteria are diverse, like other groups of animals, but are strange enough to be classified not as an animal, nor a plant, but to a group to themselves. It is a large group though, for sure. There is about ten times the amount of bacteria cells than human cells on, and in, a human body alone. Bacteria are crazy good at multiplying and reproducing. Most will breed asexually, which means that they only need one bacterium and not two. They will split themselves in two, and each half will be a new bacteria clone. If their environment is just right, some species of bacteria can double their population every ten minutes!
Bacteria come in many shapes and sizes. They can look like a bunch of balls in a line, they can be cylindrical or tube-like, some have complex shapes, and some are simpler than others. Some are like spirals, some even have tails. All of them are too small to see with the naked eye, but their size can vary depending on the species. That's not to say they are the smallest living things, viruses are even smaller. There is a difference between Viruses and Bacteria, and as a matter of fact, some viruses have been known to infect bacteria and get them sick!
In case you are wondering how those tiny creatures can give us runny noses and faulty throats, well, I can't quite answer that question specifically, because different bacteria will do different things. But I can give you some examples. All bacteria that make people and animals sick are called Pathogens. Pathogens will infect their hosts, sort of like parasites. Most will simply stay on your skin, where they can cause no harm. Some will infect the skin, but most of the bacteria that try takeovers will try from the inside. Some may find their way inside your body, and that is not a good thing.
In general, bacteria will eat at your various parts, depending on the species. Even these creatures need to eat. If they attack your throat, then your throat gets sore. They can attack other sections of your body, thus making sure that they don't work properly. Your body has its own defensive tactics, though, and sometimes it feels like the bacteria are causing some things that your body does to protect you. For instance, a runny nose is like a river where the bacteria are trying to swim upstream. A cough is like a catapult flinging bad bacteria out of your body. (Make sure you don't cough on other people, then you'll just get them sick.) Germs don't like the heat, so your body gives you a fever to kill the bacteria.
Not all bacteria are bad for you. Some live inside your stomach, and help digest all of the food that you stuff down there. Bacteria that help their hosts are called “Mutualists”. They are the kind of bacteria that don't try to kill you. Other bacteria, called predators, will actually hunt smaller microorganisms. Isn't that strange, really tiny things eating even tinier things?