Viruses offer fascinating genetics lessons
In 1898, Friedrich Loeffler and Paul Frosch discovered that the cause of foot-and-mouth disease in livestock was an infectious particle smaller than any bacteria. This was the first clue to the nature of viruses.
Viruses are responsible for some of the worst diseases inflicting mankind: smallpox, the common cold, chickenpox, measles, influenza, shingles, herpes, polio, rabies, Ebola, hanta fever and AIDS are just a few examples of viral diseases.
There are more than 80 different families of viruses, with some 500 different varieties. Most of these fortunately do not affect humans, but affect other animals and cause blights on plants.
Viruses are genetic entities that lie on the gray area between the living and nonliving.
On their own, viruses are either DNA or RNA, which carries the genetic code for the virus, surrounded by a protein coat. While isolated outside a host cell, the virus is metabolically inert and cannot replicate.
Viruses possess a unique ability to subvert the genetic processes of a living cell and turn the cell's metabolic processes into factories to make more viruses.
A virus "infects" a host cell by attaching to a specific site on the surface of the cell by way of a specific amino acid sequence on the surface coat of the virus, which is "tuned" to attack certain kinds of cells.
Some viruses enter the cell, while others only inject their genetic material into the cell. In either case, the virus "takes over" the cell and uses the cell's metabolism to assemble many copies of the original virus, which it then ejects to infect new cells. In the process, the host cell dies.
Some viruses might remain dormant inside host cells for long periods, causing no obvious changes. When stimulated, it activates, manufactures new viruses that self-assemble and burst out of the host cell, killing the cell and going on to infect other cells.
This is the case with the varicella virus that causes chickenpox. It reappears as shingles in more than 10 percent of adults who had chickenpox as children.
The human immune system is normally very efficient in identifying and destroying invading microbes.
The immune system learns to recognize and then remembers certain proteins, called antigens, on the invading microbes and builds specific antibodies that will attack them upon subsequent invasions.
Immunization works for some kinds of viral diseases such as smallpox, measles and polio, but not for others.
The influenza-A virus is especially adept at fooling the immune system because of two types of changes that take place on the protein coat of the virus.
"Antigenic drift point mutations" that result in changes in as few as two amino acids may allow the virus to sneak unrecognized past the antibodies acquired from previous influenza infections, thereby fooling the immune system.
The other mechanism of antigenic variation is "antigenic shift," which occurs when viruses bearing unique combinations of two types of proteins enter human populations from an animal host, typically birds or pigs, or by a re-assortment of proteins between animal and human influenza-A viruses.
These new strains of viruses can be virulent and are responsible for the severe, worldwide epidemics known as pandemics, such as the "Spanish Flu" that swept the world in 1918-19, killing anywhere from 20 million to 40 million people. The "Asian Flu" in 1957-58, which killed a million people worldwide, was the first known case of the influenza virus "jumping" directly from birds to humans. Although considered a pandemic, it was identified and a vaccine was developed in time to avoid a more tragic scenario.
This is one cause of concern with the increasing number of cases of avian flu reported in Asia. At present, the virus has not mutated to a form that can be passed from person to person, but there is a high probability that it will do so.
The second area of concern is that the current avian flu virus has a structure similar to the "Spanish Flu" virus. With rapid transportation, a pandemic could spread much faster than in 1918 or 1957 and conceivably could kill 100 million or more people if a vaccine cannot be developed in time, or if there is a processing problem such as what happened with last year's vaccine.
A third area of concern is that mutations, or genetic copying errors, sometimes occur during replication. Most mutations are harmful to the virus and render it unable to function.
Some mutations do not lead to dysfunctionally inert viruses but instead generate a brand-new strain of virus that might be more prolific than the original, or which might allow the virus to be transmitted from person to person instead of from birds to people.
But any single virus might generate hundreds of thousands of copies of itself, so even if half a million are rendered inert by mutations, it only takes a few that are viable to spread the new strain.
Influenza viruses can do this. As a consequence, every year there are several different strains of influenza virus that have to be identified in order to make a vaccine against the particular strain that might cause the "flu."
With the fear of bird virus mutating into a type that can be transmitted among humans, world health officials are fearing the worst.
Influenza in general is common in adults. From 5 to 40 percent of the general population gets the flu annually, depending on the severity of the influenza strains in a flu season.
According to estimates, the total economic burden in the United States alone can be as high as $12 billion in some years, due to employee absenteeism and other indirect effects of influenza, and from direct medical costs of diagnosis and treatment.
With all the complexities of the information in the human genome, it is surprisingly easy for an invading virus to subvert it.
The great mystery is how the relatively simple genome of the virus is capable of commandeering the host cell's own DNA to divert it from assembling the proteins that it normally builds and manufacturing viruses instead.
The information carried in the genome of the virus is minuscule compared to that of the host cell. Each cell uses only certain parts of its entire DNA sequence, depending on what type of cell it is, yet viruses are able to thwart the process and take over the cell completely.
On one hand, viruses are "bugs" that cause disease, discomfort and death.
On the other hand, they are fascinating subjects of genetic studies.
Since viruses can transfer genetic material between different species of host organisms, they are used extensively in genetic engineering.
A virus might incorporate genetic material from its host as it is replicating and transfer genetic information to a new host, even one unrelated to the original host. Some researchers suspect that this might serve as a means of evolutionary change, although it is not clear how important an evolutionary mechanism it is.
Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at firstname.lastname@example.org