Biology is the study of life.

Modern medicine operates using disease theory, which means that scientists operate under the assumption that all illnesses and symptoms have a natural cause called a "disease," and that it is possible to relieve an illness by removing the disease. The word "disease" refers to a natural, observable cause, particularly those caused by self-propagating units, which are often life forms. "Self-propagating" means that a thing is capable of making copies of itself - this is a characteristic of living things, but it is also a characteristic of some things which are not alive.

Disease is frequently caused by bacteria and viruses attacking human body cells. The following is the least that a person should know about these organisms and health.

What is life?

The term for any living thing is "organism," and the term for anything which is living or nonliving is a "unit." These terms are used especially when talking about microscopic things. It is not always obvious if a thing is alive or dead. This is important from a medical standpoint because infectious units often quit causing problems in the human body if they are killed. So if a doctor can determine that a particular disease-causing thing is dead, the illness is determined to be cured.

The traditional characteristics of life are these:

Homeostasis - the unit stays the same, and does not change with its environment
organization - the unit has to be put together in a certain way
metabolism - the unit eats and makes waste
growth - the unit changes with time
response to a stimulus - the unit can interact with its environment
reproduction - the unit makes copies of itself
adaptation - the type of unit undergoes evolution through reproduction

Another common way of defining life is to say that life takes unorganized things and makes them organized within itself. This refers to eating and growing.

Cell

The basic unit of complex life is the cell. All animals, plants, fungus, algae, yeast, insects, fish, reptiles, birds, and everything else that people see as living are made of cells. A typical cell size is 10 micrometers (10^-6) in diameter; a typical cell weight is 1 nanogram (10^-9). An adult human is made of 10¹⁴ (100,000,000,000,000) cells. Cells can be seen with simple microscopes which people can buy for about $10. Most life forms - other than plants, animals, and fungus - are single-cell organisms.

The easiest common factor to study between all forms of life is a particular molecule which is identical in almost all life forms. This compound is DNA, or deoxyribonucleic acid.

DNA is a chain molecule, with every link in the chain except the first link and the last link connected to two other links. Each link of the chain is an instruction for creating life. Every life form has the full instructions (complete DNA molecule) for making itself in every cell it has. The discovery of DNA was a profound discovery because DNA is every organisms' plan for life.

Bacteria

Bacteria are cells also, but bacterial cells are special. They have certain characteristics which make them noticeably different from other cells, most notably, their size. Bacteria are smaller, being 1 to ten micrometers in diameter, or 0.00001 - 0.000001 (10^-5 - 10^-6) meters in length, so non-bacterial cells are about 1000 times bigger.

Bacteria are often the cause of many common illnesses, such as diarrhea. However, most bacteria are good. 1,000 trillion (10¹⁵) bacteria are present in every adult human, versus only 100 trillion (10¹⁴) human cells. Every body is home to many bacteria, which live independently of human actions.

The surprising thing that bacteria and other cells have in common is that they both use DNA, and nature reads it the same way everywhere. There is a disease called "diabetes" which prevents a person from making a protein called "insulin." This disease can be treated by injecting insulin into a person with diabetes. The problem is that diabetes must be made according to instructions in human DNA. Scientists began to make human DNA in the laboratory by taking the instructions from human DNA and inserting them into the instructions of the DNA for a bacterial called E. Coli. Now there are E. Coli bacteria in labs which continually produce human insulin for treating diabetics. Someday, more medicine will be made like this. This procedure of changing DNA is called "genetic engineering."

Virus

A virus is a self-propagating unit but most people do not consider it to be an organism, because it can only reproduce itself through genetic engineering. A bacteria is about 100 times larger than a virus, so a virus is extremely tiny and cannot be seen except with very expensive scientific equipment.

A virus is like a DNA syringe. It floats around in blood or other fluid until it touches a cell. The virus contains DNA, and it injects that DNA into any cell, whether for a complex organism or a bacteria. When that cell gets virus DNA inside it, it reads that as instructions to make copies of the virus. Eventually, the cell becomes so stuffed with copies of the virus that it explodes, releasing viruses everywhere.

Complex cells can be killed, and bacterial cells can be killed, but viruses lack a lot of the things that make a thing alive. They are so small and simple that there is no part of them that can be attacked - the entire virus must be destroyed, otherwise it is still functional. Because they are so small, and so difficult to disable, viruses are difficult to treat.

Common examples of viruses include influenza (flu), HIV, polio, and chickenpox.