Introduction to the Viruses

Introduction to the Viruses

In 1898, Friedrich Loeffler and Paul Frosch found evidence 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, genetic entities that lie somewhere in the grey area between living and non-living states.

Viruses depend on the host cells that they infect to reproduce. When found outside of host cells, viruses exist as a protein coat or capsid, sometimes enclosed within a membrane. The capsid encloses either DNA or RNA which codes for the virus elements. While in this form outside the cell, the virus is metabollically inert; examples of such forms are pictured below.

Viral micrographs : To the left is an electron micrograph of a cluster of influenza viruses, each about 100 nanometers (billionths of a meter) long; both membrane and protein coat are visible. On the right is a micrograph of the virus that causes tobacco mosaic disease in tobacco plants.

When it comes into contact with a host cell, a virus can insert its genetic material into its host, literally taking over the host's functions. An infected cell produces more viral protein and genetic material instead of its usual products. Some viruses may remain dormant inside host cells for long periods, causing no obvious change in their host cells (a stage known as the lysogenic phase). But when a dormant virus is stimulated, it enters the lytic phase: new viruses are formed, self-assemble, and burst out of the host cell, killing the cell and going on to infect other cells. The diagram below at right shows a virus that attacks bacteria, known as the lambda bacteriophage, which measures roughly 200 nanometers.

Viruses cause a number of diseases in eukaryotes. In humans, smallpox, the common cold, chickenpox, influenza, shingles, herpes, polio, rabies, Ebola, hanta fever, and AIDS are examples of viral diseases. Even some types of cancer -- though definitely not all -- have been linked to viruses.

Viruses themselves have no fossil record, but it is quite possible that they have left traces in the history of life. It has been hypothesized that viruses may be responsible for some of the extinctions seen in the fossil record (Emiliani, 1993). It was once thought by some that outbreaks of viral disease might have been responsible for mass extinctions, such as the extinction of the dinosaurs and other life forms. This theory is hard to test but seems unlikely, since a given virus can typically cause disease only in one species or in a group of related species. Even a hypothetical virus that could infect and kill all dinosaurs, 65 million years ago, could not have infected the ammonites or foraminifera that also went extinct at the same time.

On the other hand, because viruses can transfer genetic material between different species of host, they are extensively used in genetic engineering. Viruses also carry out natural "genetic engineering": a virus may incorporate some genetic material from its host as it is replicating, and transfer this genetic information to a new host, even to a host unrelated to the previous host. This is known as transduction, and in some cases it may serve as a means of evolutionary change -- although it is not clear how important an evolutionary mechanism transduction actually is.

HIV

HIV stands for human immunodeficiency virus. It is part of a group of viruses known as retroviruses and is the third one to be discovered since 1980. To be even more specific, it is a class of retroviruses known as lentiviruses. Lentiviruses are traditionally associated with chronic arthritis and anemia and cause slowly progressive and often fatal diseases. The earliest documented case of a HIV infection was found in a 1959 vintage blood sample from central Africa.

The epidemic probably traveled down the Mombasa-Kinshasa highway in Africa and entered the U.S. during the mid 1970s. The CDC began noticing in 1981 rare diseases in gay men which traditionally affected people with damaged immune systems. They found diseases such as pneumocystis, carinii pneumonia, and Kaposi's sarcoma.

The human immunodeficiency virus infects the immune system and can lead to AIDS. People can be infected with the virus for many years without becoming ill and still transmit it to others.

The World Health Organization estimates that currently 17 million people are infected with HIV and 4 million have AIDS worldwide. By the year 2000, the number could jump to 40 million infected. More than 90% of AIDS occur in third world countries. The largest number of people infected are in sub-Saharan Africa with over 2.5 million people infected with AIDS and more than 10 million adults with HIV.

In the U.S., AIDS is mostly found among homosexual/bisexual men. 47% of reported cases in 1993 were from this group. Drug users who use needles make up about 28% of the reported cases in 1993. If you want to break this statistic down even further, you'll find that African American and Hispanics make up 54% of the reported cases in 1993. Plus, this group also has the greatest annual increase. Of heterosexuals who get HIV, 42% got it from contact with drug users. 78% of men and 74% of women who are heterosexual are African Americans or Hispanics too. Minorities account for 51% of the reported cases among adults and adolescent males and 75% in males. Of people who are less than 13 years old, 84% were minorities. In 1991, AIDS was the leading cause of death for Black and Hispanic males 25-44 and the third leading cause for women.

There is good news in this epidemic. In a survey of American sexual habits in 1994, it was found that Americans tend to have sex with partners of similar social standings. Thus it is unlikely to spread out of the poor sector. It was also found that 80% of Americans only have sex with one partner or non at all a year. However, HIV stands to be a major problem in third world countries where a majority of the population is poor.

You know you are infected if you have HIV antibodies in your blood. The virus is easily killed by alcohol and peroxide.

HIV travels from host to host via blood.

It can be passed during vaginal, , and oral intercourse. However, it is not believed that kissing passes the disease. Before 1985, transfusions used by hemophiliacs were a major source of infection. Needles used to inject drugs, tattooing, and ear piercing are also ways to transfer this virus.

Pregnant women can pass it to the baby while it is still inside the uterus or by exposure to her blood during childbirth. It can also be passed through breast milk.

It can't, however, penetrate the skin or be transmitted by blood sucking insects such as mosquitoes and fleas or spread through the air, water or food.

It doesn't produce any specific symptoms. It basically just weakens your immune system and can lead to AIDS. AIDS stands for acquired immunodeficiency syndrome. (a syndrome is an occurrence of a characteristic group or pattern of symptoms.) AIDS can appear anytime from two to fifteen years to occur.

HIV constantly undergoes rapid genetic mutations in each of its victims that leaves the immune system playing constant catch up. During the late stages of the infection, the body may lose and replace two billion CD4 lymphocyte cells a day and new viruses appear at about 100 million to 680 million a day.

There are no cures, but treatments can help people live longer.

Since the virus mutates so quickly, it is very unlikely to be a vaccine available. However, 13 experimental AIDS vaccines have been given to 1,500 volunteers in the U.S. since 1988, though federal health officials reject any type of large-scale trials due to lack of confidence in the vaccine. The World Health Organization plans to test two vaccines in Thailand and Brazil in a desperate attempt.

Prevention is still the best defense.

Influenza

Influenza is something most people get at least once and most of the time multiple times in their lifetime. It can lead to pneumonia especially in the elderly, malnourished, and people with chronic lung or heart problems.

Normally every year, about 25 to 50 million Americans catch the flu and between 10,000 to 40,000 die from it.

The virus has tons of genetic material protected by a sphere protected by layers of fat and protein. All this genetic material is like having hundreds of pieces of legos which you can make tons of different shapes with. Thus, it mutates easily and frequently. This means you'll probably never be immune from the flu.

Influenza epidemics are probably as old as human history. It is hard to trace it's path because of it's genetic symptoms.

It probably plagued Athens in 430 B.C. and destroyed Charlemagne's army in 876 A.D. There is also strong evidence of an outbreak in the 16th century too. The first recorded pandemic was in 1580 across Africa and Europe. It killed thousands in 1647 as it moved from the Caribbean to New England.

It is often known by colloquial names such as "la grippe," "jolly rant," or "the new acquaintance." It was after the epidemic of 1732-33 in the American colonies that an English doctor named John Huxham introduced an old Italian folk term which connected the colds, cough, fevers to the astrological "influence" of the stars. Thus came the popularity of the term influenza. However, it never really attracted much attention in history as other diseases stole the show – that is until the pandemic of the "Spanish flue" in 1918-19.

This pandemic is 20th century's worst and deadliest faced by any modern western society. Contrary to the name, the epidemic seemed to have started in the U.S. By conservative estimates, 21 million people died worldwide out of a billion infected. At least 12 million died in India. There were about 550,000 deaths in the U.S. 1/10th of American workers were sick in bed during the winter of 1918. 1/5th of the U.S. Army and 28% of the civilians caught the flue. About 20,000 New Yorkers died fall of that year. Inuit villages in Alaska were wiped out. Samoa lost 20% of its inhabitants. However, no blood samples were saved from this epidemic so it will never be known exactly what type of virus caused this epidemic. Epidemics like this could happen again. Influenza viruses such as the Hong Kong flu, which killed about 70,000 people in the U.S. in 1968-9, remind us that influenza should not be completely ignored.

The flu virus was first isolated from chickens in 1901, but it was recognized as so until 1955. The flu virus was also isolated from runny pig snouts in 1931. This virus became know as "swine flu"

There are three main types of influenza simply named A, B, C. Influenza A is the most common was discovered in 1933. Influenza B was discovered in 1940 and influenza C in 1947. Variants of these basic types are named after the place it first strikes. For example, there is the Hong Kong (B), New Jersey (A), and Bangkok (A). Influenza A has also undergone two antigenic shifts since 1918 in 1957 and 1968.

In 1976, Ford signed a $135 million immunization campaign against the swine flu in fear that the 1918 epidemic was back. In the end, it reached only a quarter of the U.S. population. However, it also happened that Guillain-Barri syndrome, which is a type of rare paralysis, appeared ten times as much in vaccines. The government eventually paid out another $93 million in damages, putting a permanent crimp in American pharmaceutical industry's eagerness for making any vaccines.

Virions are usually roughly spherical and about 200nm in diameter. The envelope contains rigid "spikes" of haemagglutinin and neuraminidase which form a characteristic halo of projections around negatively stained virus particles.

Influenza not only affects humans but they also infect other animals such as pigs and turkey. Not only that, but it easily crosses species. Thus, new viruses are probably created in pigs and water fowl like ducks and seagulls and then later passed on to other creatures. For example, in 1980, the virus produced an epidemic in seals and caused conjunctivitis in humans who tried to help.

The influenza virus can also last for hours in dried mucus.

Generic symptoms such as fever, fatigue, coughing, and aches.

Hepatitis

The word hepatitis means inflammation of the liver. It can be caused by a variety of organisms, drugs, exposure to toxic chemicals, and alcohol. However, viral hepatitis is the most common.

The viruses that cause hepatitis was first identified in the 1960s – 70s with the help of a study of children at the Willowbrook State School for the mentally Handicapped in Long Island, New York. Filthy living conditions in the school created an infection rate of almost 100%.

Viruses that cause hepatitis are named A, B, C, delta, and E. The hepatitis A virus (also called HAV) and the hepatitis B virus (also called HBV) are the most common.

The HAV closely resembles the poliovirus. It consists of a single strand of RNA enclosed in an icosahedral (20 sided) shell. It often shows up where sanitation and hygiene is poor. However, as with some viruses, early infection, which happen in poor countries, mean mild symptoms. Outbreaks of HAV in the U.S. have mostly occurred in day care centers, among gay men, and in connection with polluted water or uncooked food. For example, shellfish from contaminated water. It has been on a decline since 1971 when approximately 50,000 Americans were infected, mostly in the 15-24 age group. HAV also shows strong "socioeconomic" ties, or how rich you are in society. HAV infects both humans and primates. People are immune to the disease once they've caught it once.