Introduction
As an engineer, I 'm pretty conservative regarding what I consider to be possible or likely. Mass and energy balances must be satisfied. Laws of thermodynamics must be obeyed. Generating and maintaining order (life) requires energy input.
Why bother getting the biology and medicine right in science fiction stories? First, provides plenty of ways to put your characters in jeopardy. Why invent something when there is already a way to get what you want done? This avoids the " blank page syndrome." Also, don't force readers that know something about medicine and biology to suspend their disbelief when not absolutely necessary. Use science fiction stories educate readers that don't already know about a particular area of medicine and biology. This could have important social ramifications as we try to set public policy related the latest biomedical advances (like cloning). The issues raised by the latest medical technology are extremely complex. They often involve weighing potential benefits to patients against economic and moral costs to society.
I'll only address standard carbon based life forms on Earth like planets. Silicon based life and other odd chemistries (like sulfur) are possible but hard to speculate on. These could be engineered by advanced species (including future humans).
Keeping Humans Alive
Physical requirements. Total pressure > 150 mm Hg (0.2 atm, equivalent to 39,000 ft altitude), PO2 > 100 mm Hg and PCO2 < 0.5 mm Hg, food, water, temperature (0 to 100F?) and gravity (0 to 2 g? (up to 15 to 20 g's in for short periods with a g-suit)).
Nutrition Life on other planets might be eatable and nutritious if based on same chemical origins and energetically and evolutionarily favored compounds. However, some vitamins and other specific compounds might not be available.
Killing and Maiming Humans
The terms morphagen and mutagen are often misused. Morphagens change the appearance of individuals. Morphagens may or may not cause genetic changes in cells. Mutagens cause genetic changes in cells. Mutagens may or may not change the appearance of individuals. Unless mutagens cause genetic changes in germ cells, these changes generally are not passed on to later generations.
The differences between antigens and antibodies are often misunderstood. Antigens are materials recognized by the immune system. Recognition is by molecular shape and charge distribution. Proteins are the most common antigens but other classes of biological compounds can also be antigenic. Most pathogens have one or more antigenic regions on their surfaces. Antibodies are produced by the immune system (Type B lymphocytes). Antibodies are proteins that attach to antigens to inactivated them, destroy them or mark them for destruction by other immune system cells (neutrophils and macrophages).
Toxins typically act quickly. The generally effect many species (sometimes entire kingdoms). Do not increase in quantity (mass) in the environment but can spread. They are generally not passed from individual to individual except by gross contamination.
There are four types of infectious agents (pathogens) with distinctive properties and behaviors. Pathogens range in size from prions (that are infectious protein molecules) to multicellular parasites.
Prions are infectious proteins. They are the most recently discovered class of pathogens. Prions appears to be responsible for Mad Cow disease (bovine spongiform encephalopathy or BSE) and variant Creutzfeldt-Jakob disease (vCJD) and Kuru in humans. Prions are protein molecules that have undergone a change in their secondary and tertiary structures and can induce this change in other protein molecules of the same type. Prions can be transmitted orally, but are much more infectious if injected. The prions identified so far are very slow acting, taking years to take affect. The can be transmitted between species to a limited extent.
Viruses are infectious agents consisting of DNA and RNA (their genetic material), generally some protein and sometimes a lipid membrane (usually taken from their host cells). Viruses are obligate intracellular parasites. They can only reproduce in host cells. Viruses are generally species specific because that have to recognize the cells they infect and generally use their host cell's proteins to aid in their reproduction. Some viruses (RNA retroviruses) can insert themselves into their host cell's DNA. Viral infections generally take days or weeks to develop. Viral infections generally do not respond to antibiotics. Viruses reproduce and spread in the environment provided they can find suitable hosts.
Bacteria are prokaryotic infectious agents. Prokaryotes are small cells without nuclei. Bacteria are generally less particular regarding their hosts than are viruses. Bacterial infections generally take days or weeks to develop. Bacterial infections generally do respond to one or more antibiotics. Bacteria reproduce and spread in the environment, sometimes without needed a living host.
Eukaryotic infectious agents include protozoa, fungi and parasites. Eukaryotes are large cells with nuclei and other complex internal structures. Humans are eukaryotes. Eukaryotic infections in humans are often hard to treat because of similarities in the biochemistry of the pathogen and the host. Eukaryotic infections often takes days or weeks to develop. Eukaryotic infections generally do not respond to antibiotics.
Genetic diseases are transmitted from parents, but environmental factors often play are role in their development. Patents can be heterozygous carriers and not show any symptoms of the disease. Most, if not all, of the genes that can lead to genetic disease also lead to an evolutionary benefit in some circumstances.
Fixing and Improving Humans
Both somatic cell and germ cell gene therapies are being investigated. Somatic cells are the cells of the body. They are not directly involved in reproduction. Genetic changes to somatic cells are not passed to later generations. Many diseases can theoretically be treated using somatic cell gene therapy. Germ cells are directly involved in reproduction. These are the cells that, given the right conditions, develop into offspring. Genetic changes in germ cells are passed to all later generations.
Tissue engineering is currently one of the hottest areas of research in medicine. With tissue engineering techniques, new parts can be grown for humans. Skin is already available. Bone and blood may be available soon. Heart muscle and blood vessels are also being actively pursued. Ultimately, the goal is to produce complete, functional organs made up of newly grown cells.
Completely artificial replacement parts (like heart valves), mechanical limbs and artificial organs (like insulin pumps) are available and constantly being improved. Some of these devices are fully implantable while others are used outside of the body and intermittently.
Bioartificial organs are made up of cells contained in some sort of artificial structure (often to protect the cells from the recipient's immune system). These devices include features of both tissue engineering and completely artificial devices. Bioartificial heart valves are available and used routinely. Current research is directed towards the creation of bioartificial livers, pain relief devices and pancreases.
Neuro-interfaces are being investigated. The nervous system uses a combination of electrical (within individual neurons) and chemical (between neurons) mechanisms to transmit information. Current devices have biocompatibility and bandwidth problems. This is a major problem with "whole body" VR and control of artificial limbs.
The development of artificial sensory organs is being studied. Definite encoding, bandwidth and biocompatibility problems exist. Cochlular implants for the deaf are already in use.
There are major problems with maintaining or recreating consciousness and self awareness in cryogenics and reanimation situations. Memories and consciousness seem to be stored diffusely and at a very fine (sub-cellular) scale.
Medical science is working very hard to stop or reverse aging. This must be done at many scales (molecular, cellular, tissues and organs and organism). Cells seem to be programed to die (or at least to stop reproducing). This gets previous generation out of the way for the next generation. Provides the pace for evolution.
Making More Humans
Cloning. Does not result in same individual (different self due to different experiences).
Mating with other species not possible by definition. Practical limitations (like using a mix of Ford and GM parts to build a car). Some hybrids (results of near cross-species mating) show enhanced vigor.
Fully electro-mechanical humans and cyborgs. Transfer of consciousness and self awareness present very tough problems.
References and Further Reading
For background material, I suggest you visit the bookstore at a nearby college or university that has a nursing program. Look for introductory biology, physiology and pathology books.
I used the following books in preparing this paper: "Textbook of Medical Physiology, 9th ed," A. C. Guyton and J. E. Hall, Saunders 1996; "Principles of Anatomy and Physiology, 8th ed," G. J. Tortora and S. R. Grabowski, Harper Collins 1996; "Anatomy & Physiology, the Unity of Form and Function," K. S. Saladin, WCB McGraw-Hill 1998; "Biology, 3rd ed," N. A. Campbell. Benjamin Cummings 1993 (I assume a later edition of this book is available); and "Biology, the Unity and Diversity of Life," C. Starr and R. Taggart, Wadsworth 1998.
The book "Control of Communicable Diseases Manual 1995, 16th ed," A. S. Benenson (Ed.), American Public Health Association 1995 is a great resource for learning more about real infectious diseases. Caution, don't read this book if you're a hypochondriac.