by W.D. McIlveen –
All readers must surely have heard of the ongoing outbreak of the Ebola hemorrhagic fever that is occurring in several countries in West Africa. The disease is indeed a nasty one that starts with a fever, muscle pain, and headache followed by vomiting, diarrhea, and impaired kidney and liver function. Internal and external bleeding may also occur. The mortality rate of the current outbreak stands at about 70%. As of 14 October 2014, 9,216 suspected cases and 4,555 deaths had been reported. Given that the total population of the three main affected countries, Liberia, Sierra Leone, and Guinea, is over 22.1 million people, this translates to only 0.041% of the population infected. While this may seem to be a very small proportion, it must be realized that the number of people infected is continuing to increase.
Ebola hemorrhagic fever is caused by a virus. Among a large number of virus diseases affecting humans, there are some rather familiar types including AIDS, chickenpox, common cold, hepatitis, influenza, rabies, shingles, measles, SARS, West Nile fever, and yellow fever. Many of these have multiple strains. A virus is not a living organism though it does share some characteristics with pathogens of plants and animals. Instead, viruses are essentially sections of rogue DNA or RNA genetic material surrounded by a protein coating. They are capable of reproducing (increasing in number) by causing their host to make more copies of the virus, each one capable of causing the disease in the host.
All living species as well as viruses need to be able to reproduce themselves. If they don’t, then, over time, they will simply die out. If a species is to retain a stable population, then they need to replace themselves on a one for one basis. In the case of animals including humans, the female must, during her lifetime, produce two offspring (male and female on average) that live to reproduce themselves. Of course, not all offspring will survive to do so therefore the females must produce in excess of the minimum to compensate for the premature loss of the young. If she doesn’t, the species will die away. If she produces more than the number required to maintain the status quo, then the population will increase. This is simply a basic biological principle that often gets overlooked.
In general, a reproducing population will increase at rates in excess of the minimum. The rate of increase is much like the way one must pay interest on the portion of an unpaid loan or receives interest (albeit very low these days) on a bank account. The longer that such an account exists, the greater will be the amount of accrued interest. The same thing applies in biological systems and the data can be plotted mathematically in graphs (Fig. 1). The rate of increase, ‘r’, can vary widely. A plant for example may produce just a few seeds or may produce thousands of seeds each year. The larger the number of seeds that are produced, the faster that an unchecked population can grow. When a population of some species is growing at a high rate (e.g. a rate higher than we humans wish it to be), we reach epidemic or invasive proportions.
In reality, a species will typically increase until there is no more space available or all of the resources are used up (Fig. 2). Dandelions, for example, will spread in a newly cultivated field until there is no more space left in which a seed can germinate and grow into a new plant. Disease organisms such as potato blight will spread through the crop until it has infected all of the available host foliage. The same principle applies to all organisms – fungal, bacterial, viral, insect, mammal, etc. Populations simply cannot increase forever. As well as exhausting all available resources or space or hosts, other factors will often have a bearing on the situation for they too will become hosts for a different type of organism. For example, an infestation of caterpillars on a crop will be subjected their own set of parasites and hyper parasites that keeps the population in check. And it is never in the long-term interest of a parasite to kill all of its hosts.
In reality, not every seed will germinate, or the conditions for growth are not suitable, or the plant or whatever organism is itself subject to its own parasite. In a balanced natural system, the growth of populations of all types of biological entities is constrained and the system functions as nature intended. The problem comes when a particular species does not have its inherent control systems. This is usually the case (initially) for a newly introduced species and it reaches undesirable numbers. This applies to plants, insects, fungi, bacteria, viruses, etc. But the net effect of all the factors that may come into play are keeping the ‘r’ rate lower.
Now, in relatively recent times, the computer programmers needed a word to apply to situations when malicious software was introduced into the electronic communications used by functioning computers. They latched on to the term ‘virus’ from the field of biology for they considered the desirable programs running on the computer to be ‘infected’ by the undesirable software. They could have just as easily used the term ‘fungal’ or ‘bacterial’ or even ‘epidemic’ to indicate the ability of the software to spread to other computers. The application of the term ‘viral’ still remains incorrect. Even more recently, people have used the term ‘viral’ to indicate the rapid spread of a photo, film clip, or some type of information among cell phone users and the like. The use of the word in this way is quite different from that where the malicious software is involved. It is even more distant from the true meaning of ‘viral’ and its use should be discouraged. Hopefully, time will cause the term to become obsolete in this manner of use and that its use will return to where it truly belongs – restricted to the world of virologists that are dealing with the spread of real issues including infectious diseases such as Ebola.