The virus uses the host cell machinery to cause the host cell to manufacture additional virus particles. For this reason it is questionable as to whether viruses are actually living organisms. Because of this, viruses can only be visualized with the help of electron microscopes. The two cell types found in nature are prokaryotes and eukaryotes . The former constitute many of the single celled life forms found in the microscopic realm.
Bacteria are the smallest all cell types, and have a very simple cellular structure.
On the inside, bacteria have circular double-stranded DNA and a watery cytoplasm in which are found metabolic enzymes, water, food and waste molecules, and so on. Bacteria are capable of extremely rapid growth and cell replication, multiplying at geometric growth rates, making them useful as future food sources, and making them menacing when they invade human or animal tissues. Eukaryotes are much larger than prokaryotes, generally about 100 times larger. Look at the illustration below of a eukaryote that lives its life as a single cell, swimming around in the water of the command pond. It is a cell that gets its energy from the sun, that has a whip-like structure that extends from its cell body that helps it move.