The first forms of natural life on Earth were tiny Micro-Organisms (bacteria and viruses) before they started mutating and growing into more complex life forms. Long before the first living creatures existed on this planet, long before plant-life, micro-organisms were around, converting the atmosphere into one that could harbor life as it is today. Micro-organisms can survive in environments much harsher than what animals and plants can cope with. They've been know to cope in extreme temperatures such as in volcanic vents. There are more different types of viruses and bacteria than you could imagine. Whilst most are harmless, others ravage the hosts and cause disease. The human body consists of many thousands of these micro-organisms, some performing useful tasks, others are more dangerous, asleep, waiting to be activated.
Bacteria is a complete living, self-reproducing micro-organisms that contains DNA and RNA. A virus on the other is nothing more than a few bits of DNA or RNA protected by a protein shell. They cannot reproduce naturally but must find a host in which to replicate. Once they have found their way into a host cell, they use their hosts D/RNA to fill the gaps in their instruction set. Once they have the necessary instructions, they destroy their host cell and move on. They enter as a single virus and when the host cell explodes, thousands of its copies are released. Not all Bacteria is harmful to humans, in the gut we have bacteria which helps us to digest food. Both types can cause disease but are treated in different ways. Bacteria can be killed by Anti-Bacterial drugs but for Viruses, whilst there are some anti-viral drugs, they just run their course. Injections are used to create immunity to diseases such as Polio, Tuberculosis and Influenza Viral Disease. More information on the differences can be found at Net Doctor and Diffen.
As viruses don't have all the materials they need to reproduce like bacteria, they frequently mutate which makes fighting them all that much more difficult. Viruses have been known to mutate and cross the species boundary such as in the case of swine flu. RNA viruses mutate more quickly than DNA viruses because of errors in the coding. CBC. Its beyond the subject matter to list all the differences between RNA and DNA except to say that RNA are smaller, cut down version of DNA and needs information from the DNA to replicate. If you're interested in reading more, I can suggest Diffen, its a different page to the one above. RNA viruses mutate more because they use more of the host cells instructions inside them.
As microorganisms have been known to survive in extreme temperatures ranging from the very cold to the very hot. We can't discount the possibilities that micro-organisms exist on our nearby planetary neighbours of Mars and Venus. Whilst most people look at Mars as possibly having some form of micro-organisms, we should also look at the harsh environment of Venus. As its difficult to land anything on the planet of Venus, there is not so much interest in that planet as to whether there is micro-organisms on that planet.
In 2003, Prof. Nalin Chandra Wickramasinghe of Cardiff University wrote a letter to the Lancet medical journal that said the SARS virus could've come from outer space having somehow survive entry into the atmosphere. It is something that's not yet been discounted or accepted as fact yet. The theory (Panspermia) that human DNA hitch-hiked on a comet here is nothing new, Wickramasinghe altered the theory slightly. The Invasion of the Bodysnatchers series of films play on the fact that alien viruses travel through space and infect people on Earth.
As humans explore and expand their landspace because of the population increasing, we are going to come into contact with new diseases. Over the last hundred or so years, we have seen the emergence of viruses such as Spanish Flu, H5N1, Ebola and HIV. Spanish Flu appeared soon after the First World War, it killed more people than those who died in the war. Ebola is a virus that keeps turning up now and again in Central Africa. It is named after the river where the first cases were discovered. It kills its hosts too quickly for it to become a world wide threat but we shouldn't be complacent. H5N1 is causing a lot of interest in the media because they don't know too much about the virus. It should be noted that people are surviving the H5N1 naturally without drugs and it is not the killer disease that will cause mass-extinction on this planet. There is a fear amongst some people that the Smallpox virus which many people have long thought wiped out could make a re-emergency again. It isn't just hot areas where new micro-organisms could emerge but the cold ice and snow wastelands of the Arctic and Antarctic. Trapped inside those ice could be a new virus waiting to come out. As the world warms up because of global warming, these viruses will escape. Viruses can lie frozen for many years then once thawed can be as deadly as they were before they froze.
Whilst some micro-organisms can survive outside a living host, others cannot. The examples contained here are not the only way these micro-organisms can travel, some can travel more than other ways. Those that cannot need to either travel by touch (in the form of common cold (amongst other ways), water (such as Cholera), bodily fluid transmission (AIDS), through bites (Rabies, Malaria). Simple hygiene techniques can be employed to help prevent the spread of these infections. For advice on preventing micro-organisms from spreading, I recommend the dedicated N.H.S. page.
Unnatural bacteria and viruses in a host can cause disease and sometimes death. Whilst most micro-organisms will cause nothing more than the common cold, other viruses will cause much worse such as the Varicella-Zoster virus which causes Chickenpox and Shingles. Inappropriate use of medicines can make these diseases resistant to what is supposed to kill them. In hospitals, we are seeing cases of MRSA (Methicillin-Resistant Staphylococcus Aureus) which is resistent to almost all forms of detergents. Causes of these viruses becoming immune are :- Inappropriate overuse of drugs i.e. using when not required, Weak use of detergents to clean places and their inappropriate us, People who don't take the complete course of drugs that are prescribed to them are also at risk of making viruses stronger. Even if you feel fine, the virus might still be in the body. Another worrying disease making a return is drug-resistant TB. This is by no means an exhaustive or detailed explanation of viruses, you read further on them.