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VIRUS DISEASES
Viruses cause many diseases in humans, animals & plants. Among the more common viral diseases
are the common cold (Rhinovirus), chickenpox (Herpes Zoster Virus) & coldsores (Herpes Simplex
Virus). 3 important diseases caused by RNA viruses are: Polio, Influenza & HIV/AIDS.
POLIO:
Viral poliomyelitis is now very rare in developed countries (success of vaccination programs) but
still a common illness in developing countries.
Polio enters its host via the oral route through contaminated water, food or saliva & replicates in the
B- & T-cells in the small intestine. Unless there is a strong immune response, the virus enters the
blood stream & in about 1% of those infected the polio virus attacks motor neurones & the CNS
causing paralysis or death. Infectious polio virus is excreted in the faeces where it can spread to the
sewage & water supplies.
The polio virus is a small RNA-containing virus with a simple
spherical structure shown here in a 3D reconstruction based on E.M.
pictures & X-ray crystallography data.
2 different types of polio vaccine are in widespread use & have saved
literally hundreds of thous&s of lives. The Sabin virus is a highly
weakened (attenuated) form of the polio virus which cannot
replicate in human nerve cells & so cannot cause paralysis. The
Sabin polio virus replicates slowly in the intestine ensuring the rapid
generation of a strong host immune response. The Salk vaccine in
contrast uses a preparation of killed polio virus which can not
replicate in the host but is able to give rise to an immune response which will protect the host against
a later polio virus infection.
INFLUENZA :
Influenza (or 'flu) is a serious respiratory illness which has killed millions of people. Influenza A
viruses can infect man, pigs, chickens, ducks, seals & horses. The influenza virus is transmitted by
inhalation of droplets & the virus attaches itself to the cells which line the inside of the throat &
lungs.
Electron micrograph of Influenza virions. Influenza virions contain
RNA molecules surrounded by a viral proteins & a layer of
membrane taken from the infected host cell.
The influenza virus outer surface has spikes which consist of two
virus encoded proteins which are important for entry into host cells.
Small changes (or mutations) in the amino acids which make up the
proteins occur all the time. The constantly changing amino acid
sequence of the influenza virus coat proteins leads to the emergence of new strains of Influenza
virus. These new strains can replicate in individuals who mounted a strong immune response to
previous strains of influenza. When a new 'flu strain emerges which most of the population has no
effective immunity to the stage is set for a global influenza epidemic (or pandemic).
HUMAN IMMUNODEFICIENCY VIRUS HIV/AIDS
The Human Immunodeficiency Virus (HIV) is a retrovirus. The genetic information in a retrovirus
particle is encoded by RNA. Upon entry into the host cell this RNA is copied into DNA by the virus
enzyme reverse transcriptase. This cDNA copy of the virus' genetic information can integrate into
the host cell chromosomes in the nucleus. This provirus can lay dormant for many cell divisions
before being reactivated & producing more infectious retrovirus particles.
A schematic representation of an HIV virion.
HIV infects cells of the immune system, most notably macrophages & CD4 positive T-cells. The
HIV virus is transmitted via contaminated blood, for example in blood transfusions prior to 1986 &
by contaminated syringe needles. HIV is also transmitted by heterosexual & homosexual intercourse.
After gaining entry to the host the HIV virus infects macrophages & once the infection is established
virus is released into the bloodstream where it can attack macrophages & CD4+ T-cells. There is
often a brief period of illness & fever which is followed by the appearence of anti-HIV antibodies in
the blood. A positive HIV test detects these anti-HIV antibodies in blood samples. It can take as long
as six months after exposure to HIV before a person tests positive for anti-HIV antibodies.
People who are HIV positive stay in seemingly good health for several years before the first signs of
Acquired Immuno-Deficiency Syndrome (AIDS) appear. During this asymptomatic phase of the
illness, which can last for between one & ten years, HIV is replicating all the time in macrophages &
CD4+ T-cells but the body is able to replace all the cells lost through virus infection. During this
time the HIV infected person can transmit the HIV virus through blood or sexual contact.
One of the key questions faced by HIV researchers is what is happening to the virus & the immune
system during this long asymptomatic phase of the disease.
One of the first signs of illness is chronically swollen lymph nodes but it is only when CD4+ T-cell
numbers drop dramatically that the infected person's immune system begins to fail & they are no
longer able to fight off infections caused by bacteria, fungi & other viruses. In full blown AIDS the
HIV virus can infect a wider range of cell types including cells in the brain & central nervous system
causing neurological disease & dementia. Additionally, several types of cancer are associated with
late stage AIDS infection.