In humans they can cause smallpox, chicken pox, influenza, shingles, herpes, polio, ebola, AIDS, rabies, and some types of cancer. The four main types of biological entities described above share some unique characteristics that can allow us to distinguish them from non-living things. Organisms tend to be complex and highly organized. Chemicals found within their bodies are synthesized through metabolic processes into structures that have defined purposes. Cells and their various organelles are examples of such structures.
Cells are also the basic functioning unit of life.
Cells are often organized into organs to create higher levels of complexity and function. Living things have the ability to take energy from their environment and change it from one form to another. This energy is usually used to facilitate their growth and reproduction. We call the process that allows for this facilitation metabolism . Organisms tend to be homeostatic .