We have seen that the entire diversity seen in the living world can be classified into a few groups. This classification is based on common characteristics between different organisms. Organisms that can cause disease are found in a wide range of such categories of classification. Some of them are viruses, some are bacteria, some are fungi, some are single-celled animals or protozoans. Some diseases are also caused by
multicellular organisms, such as worms of different kinds.
which can cause acne. The scale of the
image is indicated by the line at top left,
which is 5 micro metres long.
organism responsible for sleeping
sickness. The organism is lying next to
a saucer-shaped red blood cell to give
an idea of the scale.
organism that causes kala-azar. The
organisms are oval-shaped, and each
has one long whip-like structure. One
organism (arrow) is dividing, while a cell
of the immune system (lower right) has
gripped on the two whips of the dividing
organism and is sending cell processes
up to eat up the organism. The immune
cell is about ten micrometres in diameter
lumbricoides) from the small intestine. The ruler next to it shows four centimetres to give us an idea of the scale.
Common examples of diseases caused by viruses are the common cold, influenza, dengue fever and AIDS. Diseases like typhoid fever, cholera, tuberculosis and anthrax are caused by bacteria. Many common skin infections are caused by different kinds of fungi. Protozoan microbes cause many familiar diseases, such as malaria and kala-a-zar. All of us have also come across intestinal worm infections, as well as diseases like elephantiasis caused by diffferent species of worms.
Why is it important that we think of these categories of infectious agents? The answer is that these categories are important factors in deciding what kind of treatment to use. Members of each one of these groups – viruses, bacteria, and so on – have many biological characteristics in common.
All viruses, for example, live inside host cells, whereas bacteria very rarely do. Viruses, bacteria and fungi multiply very quickly, while worms multiply very slowly in comparison. Taxonomically, all bacteria are closely related to each other than to viruses and vice versa. This means that many important life processes are similar in the bacteria group but are not shared with the virus group. As a result, drugs that block one of these life processes in one member of the group is likely to be effective against many other members of the group. But the same drug will not work
against a microbe belonging to a different group.
As an example, let us take antibiotics. They commonly block biochemical pathways important for bacteria. Many bacteria, for example, make a cell-wall to protect themselves. The antibiotic penicillin blocks the bacterial processes that build the cell wall. As a result, the growing bacteria become unable to make cell-walls, and die easily. Human cells don’t make a cell-wall anyway, so penicillin cannot have such an effect on us.
Penicillin will have this effect on any bacteria that use such processes for making cell-walls. Similarly, many antibiotics work against many species of bacteria rather than simply working against one.
But viruses do not use these pathways at all, and that is the reason why antibiotics do not work against viral infections. If we have a common cold, taking antibiotics does not reduce the severity or the duration of the disease. However, if we also get a bacterial infection along with the viral cold, taking antibiotics will help. Even then, the antibiotic will work only against the bacterial part of the infection, not the viral infection.