I know that a bacterial infection is pretty much curable with an antibiotic. My understanding is that viral infections are not curable (although some are preventable through a vaccine). What is the difference between the two organisms?
Most pathologists don’t consider viruses to be “organisms”, in the traditional sense of the term. While bacteria are basically cellular in structure, although normally lacking a clearly-defined nucleus, virii are snippets of RNA or DNA encased in a protein capsule. While bacteria are completely capapble of self-support and replication in the right environment, virii are totally dependant on being able to take over the reproductive chemistry of a living cell in order to replicate. In general, they do this by injecting their RNA or DNA payload into the host cell, which then procedes to use it’s own reproductive chemistry to make copies of the virus. Eventually, when enough virii have been produced, the host cell bursts open and the new virii go on to infect more cells. Normally, the host animal’s immune system can eventually muster enough of it’s forces to repel and stop the attack, but some virus infections are too much for the host, and the infection eventually kills it.
http://www.mult-sclerosis.org/bacterium.html
Bacteria are single-cell organisms. They are neither animals, plants nor fungi but belong in a group all of their own. They are not the same as viruses which are considerably smaller and simpler structures and, unlike bacteria, are unable to reproduce on their own. Bacterium is the singular of the word and bacteria is the plural - one bacterium and many bacteria.
Bacteria are best known for the diseases that they cause but most bacteria do not cause diseases or depend on other organisms for their existence. Bacteria were the first organisms to evolve on the planet, around three thousand million years before there were any multi-celled animals.
One of the defining features of bacteria is that their DNA is not enclosed in a nucleus like plants and animals. Bacteria reproduce asexually by simple cell division called mitosis though they have other ways of exchanging DNA material. This enables them to reproduce very quickly and one bacterium can become several million bacteria in a very short space of time if the conditions are right.
Some bacteria contain chlorophyll and can synthesise their own sugars from sunlight and carbon dioxide much like plants.
Some bacteria live in the guts of large animals, including humans, and are essential to that animals digestive processes.
Some bacteria can survive in boiling water and others can survive radiation more a thousand time that which would kill people.
A number of bacterial species cause diseases in animals. A variety of the Chlamydia bacteria, C. Pneumonia which can cause pneumonia in weakened individuals has been linked with multiple sclerosis. This link has been challenged by many other researchers.
http://www.mult-sclerosis.org/virus.html
Viruses (virii) are strands of DNA or it’s complimentary form, RNA, usually coated by a protein shell called the capsid. Viruses lie at the very cusp of life - they cannot replicate themselves on their own and need to coerce a host to make copies of themselves. Because of this they need to get inside the cells of living organisms - animals, plants, fungi, plasmodiums or even bacteria.
It is important to distinguish between viruses and bacteria. Bacteria are fully functional, single-celled life-forms which are capable of replicating themselves on their own. Whilst both viruses and bacteria can cause diseases, the ways in which they do this are very different. Unlike bacteria, viruses do NOT respond to antibiotics. Viruses are many times smaller than both bacteria and human cells. Although many bacteria cause diseases, most of them do not live inside the bodies of other organisms and do not cause diseases.
There are three kinds of virus,
DNA viruses which carry regular DNA much like that in the nucleus of human cells. These include the herpes family, smallpox, and the papilloma family of viruses which cause warts and benign and malignant cancers (it is estimated that the papilloma viruses may cause 16% of cancer in women and 10% of all cancers worldwide).
RNA viruses which carry a substance called RNA. In animal and plant cells, proteins are synthesised from DNA via an RNA stage. RNA viruses parasitise the normal functioning of the cell by operating in this RNA phase of protein synthesis. Retro-viruses are themselves sub-divided into
plus-stranded RNA viruses, eg. polio virus and the flavivirus family which causes yellow fever, dengue, and various forms of encephalitis.
negative-stranded RNA viruses, eg. influenza, measles, mumps and rabies viruses.
double-stranded viruses, eg. the Epstein-Barr and Colorado tick fever viruses.
Retro viruses which carry RNA but copy this back into DNA when they are operating with the organism’s cell. Examples of retro-viruses are HIV which causes AIDS and HTLV-1 which infects T-cells and causes Tropical Spastic Paraparesis, a differential diagnosis of Multiple Sclerosis.
Many viruses have been implicated in the development of multiple sclerosis. Currently, it is Human Herpes Virus 6 (HHV6) that is the focus of attention but it may very well be that viruses have nothing whatsoever to do with the etiology of MS.
Upper respiratory chest infections, some of which are associated with viral infections, have been shown to be positively correlated with MS relapses. However, these may exacerbate MS by activating the immune system through the release of pro-inflammatory cytokines and may not be directly related to the disease.
A bacterium is an organsism. That means it’s a life form based on cells (in this case, each bacterium is one cell) which engages in the life processes of ingestion, digestion, excretion, respiration, reproduction, etc. They can grow and reproduce independent of other living matter.
Viruses are not complete cells. They cannot build themselves, and can only reproduce by invading a host organism and using some of its capabilities.
Here are some links that discuss it:
http://vm.cfsan.fda.gov/~dms/qa-fdb38.html
http://www.microbe.org/microbes/virus_or_bacterium.asp
If a virus is not an organism in the traditional sense, then what does it mean for a vaccine to be prepared with a “dead” virus?
Why are there vaccines for viruses but not bacteria? (Follow-up: Why does contracting some viruses confer immunity but you can get the same bacterial infection a zillion times?)
What is the difference between how our bodies fight bacterial vs. viral infections?
As usual on this board, I know that the answer to these questions probably fill a couple of hundred biomedical texts, but just give me the Cliff’s Notes version 
Stictly speaking, vaccines are not prepared from “dead” virii, they are prepared from weakened ones. The weakened virii are not capable (in most instances) of infecting live cells, however they do provoke the same immune respose that a fully functional virus would. Thus, when an immunized persone gets exposed to the full-strength virii, her immune system is prepared to fight it better, having “remembered” from it’s encounter with the weakened vaccine virii.
Cliff-notes enough for ya?
A dead virus isis a virus that can’t reproduce, either because its DNA/RNA has been damaged, or because it is missing.
Our bodies fight bacterial and viral infections in the same way. First, white blood cells called T-cells engulf and digest the bacteria or virus. Secondly, other cells create antibodies…chemicals designed specifically to the invading virus or bacteria, that kill any future bacteria or virii of the same kind.
And, there are vaccines for some bacteria. There’s a vaccine for bacterial pneumonia, and one is being designed for tuberculosis, for example.
Flu and polio vaccines are prepared from dead virii, but most vaccines aren’t. Live vaccines are more effective than dead ones.
My microbiology degree was long ago and almost never used any more, but I can at least give some (nit-picky) help with definitions.
“Virus” is a plural term, the singular form is virion (“virii” as a word isn’t correct although it is used). Referring to dead and weakened virus is also a layman’s term. The correct term is inactivated and attenuated, respectively.