Germs are microorganisms that cause disease. Bacteria are living organisms with their own metabolism. Most live in harmonious co-existence with our pets and with us but some, called "pathogenic bacteria", cause illness and disease. Generally speaking, bacteria multiply when mature cells divide into two carbon copy juveniles. They can do so almost anywhere. Like all living cells, each bacterium consists of a nucleus (without a containing membrane) and a cell wall. A unique type of extremely small bacteria are called rickettsiae. Unlike all other bacteria (and more like viruses), rickettsiae can only multiply inside living cells.
Fungi are also living organisms, with rigid cell walls. They need organic material to feed on. In the outside world fungi usually exist as multi-celled thread-like mould, but they can also live in a single-celled yeast form. Yeast multiply by forming new bud-like spores. Under certain conditions some fungi are pathogenic in their yeast form.
Viruses are the smallest infectious agents. Basically, a virus is no more than a package of genetic material encapsulated in a protective protein shell. Viruses cannot survive for long or multiply outside living cells. A virus multiplies by invading a cell and taking over the cell's reproductive apparatus. Some viruses cause little damage to their "host" cells but others are powerfully destructive, wiping out the cells they multiply in. Viruses are, in effect, nature's smallest and most efficient parasites.