MadSci Network: Virology

Re: How come AIDS is so dangerous?

Date: Sun Nov 23 16:44:48 2003
Posted By: Joshua Chai, Student Doctor, School of Clinical Medicine, University of Cambridge, UK
Area of science: Virology
ID: 1069036630.Vi

Hi Emma,

Thanks for your question. AIDS is caused by Human Immunodeficiency Virus (HIV). It is no different to many other viruses that cause human sufferings in the sense that modern medicine cannot offer an effective cure. Broadly speaking, human diseases can be caused by internal or external factors. Among the external factors are infectious micro- organisms. To give you a simplistic view, infectious organisms can be one of 3 types: (i) parasites - which could either be metazoa (multicellular) or protozoa (unicellular); (ii) bacteria; and (iii) viruses.

When parasitic or bacterial infectious organisms invade our bodies, they do so as "foreign" cells with their unique cellular machinery different from our own. It is this difference between the cellular machinery of the invading organisms and that of own that allow drugs to work selectively. Rather like a battlefield, drug molecules need to be able to target the invaders selectively, distinguishing them from the self, in order to eliminate the threat without harming the host. To give an example, the well-known antibiotic penicillin (a naturally-occurring chemical) inhibits the synthesis of cell-wall which is unique to some strains of bacteria. All the cells in you and my body consist of only cell membrane but no cell wall. Therefore targeting the cell-wall will do us no harm! The same principle of selective targeting applies to most of our pharmacological weaponry against infectious micro-organisms.

However, viruses are very different. They do not have cells. They consist of a core of genetic material (DNA or RNA), a protein coat called capsid, and sometimes an outermost lipid bilayer envelope. They are very simple agents that effectively "hi-jack" the infected cells' machinery to produce viral proteins for them. Therefore we say viruses are obligate intracellular micro-organisms: they cannot replicate outside living cells. This posts big problems. First, we no longer have an obvious and unique cellular machinery to selectively target; second, when infected, the viruses hide inside the host cells hence our immune system cannot see and destroy them. So, what can we do? Are we going to be all killed by exotic viruses? Luckily, the answer is an obvious no...

Going back to your question, you correctly mentioned that some viral infections are indeed very nasty and dangerous e.g. Ebola, SARS, Rabies. But our immune system are so clever that it has means (beyond the scope of this simple account) to seek out virally infected cells and destroy the infected cells together with the viruses hiding inside them. So may be viruses are not so dangerous after all. In fact, for some specific viral infections e.g. herpes simplex virus (causing chicken pox and shingles), there are some very effective drug treatment that would clear the skin rash in a matter of a week or two.

In that case, why are people so afraid of HIV infection? The answer lies in the fact that HIV infects specifically a certain types of white blood cells crucial to the function of our immune system. You can imagine that once infected, the immune system is gradually impaired, until a point when the immune system is so weak that the infected person cannot defend him/herself against any other opportunistic infections. And it is true that at the moment there is no effective cure for HIV infection. To make matters worse, most infected individuals are unaware of the infection initially and they can easily spread the disease to others by blood/sexual contact.

Therefore, to summarise, HIV is essentially no more dangerous than other infectious agents (Indeed, parasitic and bacterial infections do kill a very large number of people each year worldwide!!). HIV mainly kills by impairing the host's immune system and making them susceptible to other nasty infections e.g. pneumonia. It deserves global attention not because it is the most dangerous virus of all kind, it is because it has a long incubation period (i.e. infected but showing no symptoms of it) and hence can be spread silently across a vast population in a relatively short period of time, as was the case in part of Africa and South Asia. Without effective treatment, as it is the case at the moment, many lives are going to be lost. Fortunately, with careful handling of blood and blood products, and very importantly global awareness of the practice of safe sex, the chance of catching this notorious virus in developed countries is not as high as you might think.

I hope this helps clear some of the myths about AIDS!

Joshua Chai
Student Doctor, School of Clinical Medicine, University of Cambridge, UK

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