Germ Warfare Among the Lower Eukaryotes

Paramecium is a well-known ciliated protozoan. Many strains of Paramecium carry symbiotic bacteria ( Caedibacter ), known as kappa particles , that grow and divide inside the larger, eukaryotic, Paramecium cell.
Those strains of Paramecium with kappa particles are known as killers.
Killing occurs when kappa particles are released by a killer and eaten by a sensitive Paramecium (i.e., one lacking kappa particles). Digestion of the kappa particle results in release of a protein toxin and death of the sensitive Paramecium . Thus germ warfare is practiced by single-celled eukaryotes.

The toxin gene is borne not on the chromosome of the symbiotic bacteria, but on a bacterial plasmid derived from a defective bacterial virus. So a toxin encoded by a virus infecting the kappa particle bacteria has been diverted to the purpose of killing other strains of Paramecium . This appears to be a general principle. Many of the toxins used by pathogenic bacteria that infect humans are actually encoded by DNA of nonchromosomal origin—viruses, plasmids, or transposons. These elements are often integrated into the chromosome of pathogenic strains of bacteria.

Some insects rely on viruses to wage biological warfare. Certain parasitic wasps inject their eggs into plant-eating maggots (i.e., the larvae of plant-eating insects). After the eggs hatch, the newborn wasps eat the living maggots from inside. The maggots are eventually killed and a new generation of wasps is released to continue their life cycle.

The secret to the wasp’s success is injecting a virus along with their eggs.  The virus, a member of the adenovirus family, targets the maggot’s “fat body” (vaguely equivalent to the liver of higher animals). The virus cripples both the maggot’s developmental control system and its primitive immune system. The maggot loses its appetite for plants and is prevented from molting and turning into a pupa, the next stage on the way to an adult insect capable of laying more eggs. There are many types of plant-eating maggots, including major agricultural pests such as the tobacco budworm and cotton bollworm, and many types of wasps that attack them, and consequently many types of viruses, each designed to soften up a particular insect larva.


Burning crops was probably the earliest form of warfare aimed at undermining an enemy’s survival by biological means. Early in history, the water supply was also a prime biological target for feuding nomads, especially in areas where water was scarce. Presumably tossing dead or rotting animals into waterholes poisoned the drinking water and proved to be reasonably effective in driving the enemy away.

Throughout history there have been occasional sporadic attempts to deliberately spread infection for military purposes. However, these have mostly been ineffective or irrelevant. During the Black Death epidemic of the mid-1300s, the Tartars catapulted plague-ridden corpses over the walls into cities held by their European enemies.

Although this is sometimes credited with spreading the plague, in reality, rats and their fleas spread bubonic plague , not contact with corpses. Catapulting bodies into a city may deserve points for enthusiasm, but it doesn’t earn an A in microbiology. In medieval Europe, dead or sick animals were hurled over the walls into castles or walled cities to break sieges by spreading disease. Nonetheless, given the state of hygiene in most medieval towns or castles, there was often little need to provide an outside source of infection. With plague, typhoid, smallpox , dysentery, and diphtheria already around, all that was usually necessary was to let nature take its course. Similarly, attempts of white settlers to spread smallpox among the American Indians were not only rather ineffective but also largely irrelevant because smallpox had already spread by itself.

The reason why germ warfare has been of little account until recently is that plenty of dangerous infections were already in circulation. If an army was crowded and unhygienic, some natural disease would undoubtedly attempt a biological assault without waiting for artificial prompting. Until recently, armies, like civilian populations, were so dirty and disease-ridden that practicing germ warfare was rather like trying to kill a shark by drowning it. Only in our modern disinfected age has spreading disease deliberately become a meaningful threat.


During the Vietnam War, the Viet-Cong guerillas dug camouflaged pits as booby traps. Within these they often positioned sharpened bamboo stakes or splinters smeared with human waste. Although it was possible to contract a nasty infection from these, the main purpose was psychological. Worrying about possible booby traps hampered the movements of American troops out of all proportion to actual casualties. Thus the threat of chemical or biological warfare may have great psychological effect. An example is the recent anthrax bioterrorism in the United States. There have been several times as many fatalities from the naturally spreading West Nile virus as from the deliberate delivery of anthrax spores. Yet response to the anthrax scare has involved colossal disruption of the postal service and massive expense.

Taking protective measures against a possible biological attack is costly and inconvenient. Vaccinating soldiers against all possible diseases that might be used is impractical. In addition, vaccines sometimes have side effects, especially if they have been developed under emergency conditions without thorough testing. Consider the anthrax vaccine used by the U.S. army that was approved in 1971. It has been thoroughly tested and is regarded as relatively safe. Vaccination requires six inoculations plus annual boosters. It produces swelling and irritation at the site of injection in 5% to 8% and severe local reactions in about 1% of those inoculated. Major systemic reactions are “rare.” Although it works against “natural” exposure, it is uncertain whether it would protect against a concentrated aerosol of anthrax spores. Dressing infantry in protective clothing and respirators hampers their mobility, making them easier targets for conventional weaponry. In hot climates extra clothing may also promote heat stress.

Even without deliberate germ warfare, troops from hygienic temperate nations are at a major disadvantage when operating in tropical Third World situations. Drugs given to ward off malaria and other endemic tropical infections are costly, rarely 100% effective, and may damage health if taken over a long period. Constant exposure to insecticides to kill mosquitoes, lice, and so forth may damage the nervous system. An additional factor is that the inhabitants of rich Western nations expect to live into their seventies or eighties nowadays. Consequently, those sent into backward areas of the world demand ever-increasing levels of protective equipment and medication. Military actions in Third World nations are thus becoming ever more expensive. In contrast, troops belonging to a poverty-stricken local regime will go unencumbered by the extra protective gear that they cannot, in any case, afford. Moreover, human casualties are of much less importance to the regimes of overcrowded nations where life expectancy is much lower. Over the past half century, armed interference in the Third World by nations such as the Soviet Union and the United States has steadily become less enthusiastic and less effective, at least in part because of these trends.


The basic strategy of germ warfare is to choose a human disease and use it to eliminate or frighten away human competitors. Perhaps the disease agent could even be improved by genetic engineering, as discussed later. However, several major factors need to be considered. The relative importance of these factors will depend on whether biowarfare agents are intended for use on a military scale or by terrorists.

Incubation Time

One major problem with biological warfare is that death from infectious diseases is slow. Even the most virulent pathogens, such as Ebola virus or the pneumonic form of Black Death, take a few days to kill. Thus infected personnel would still be capable of fighting for a significant period. In contrast, conventional weapons kill or incapacitate rapidly.


Another drawback to germ warfare is the problem of delivery. Most protocols for spreading infections deliberately involve some sort of airborne delivery. This tends to place delivery at the whim of the weather. Not only do you need a breeze, but also the wind needs to blow in the right direction! During the 1950s the British government conducted field tests with harmless bacteria. When the wind blew them over “healthy” farmland, many of the airborne bacteria survived the trip and reached the ground alive. In contrast, when the wind blew the bacteria over industrial areas, especially oil refineries or similar installations, the airborne bacteria were almost all killed. In the event of aerial delivery, even if the wind is in the right direction, most of the population of an industrial nation will be found in cities where they will be at least partly protected from airborne bacteria by air pollution!

In addition, many infectious agents are sensitive to desiccation and become inactive if exposed to air for significant periods of time. Moreover, natural UV radiation from the sun also inactivates many bacteria and viruses. Thus most biowarfare agents must be protected from this “open air factor” before use and then dispersed as rapidly as possible.

Dispersal by terrorists involves rather different considerations. Delivery of small batches of biowarfare agents can be performed individually in a variety of ways—for example, by mailing letters containing anthrax spores. The minimalist dispersal scheme is to infect a volunteer and have him or her use public transport among crowded target populations.

Persistence of the Agent

Once biological weapons have been successfully used, the victor presumably wants to move in and take over the enemy territory. This may involve exposing the invading troops to the infectious agent, depending how long it persists in the environment. Anthrax is a favorite biological weapon because it acts rapidly and is highly lethal. Unfortunately, Bacillus anthracis, which causes this disease, spreads by forming spores that are tough, difficult to destroy, and last for an extremely long time. When suitable conditions return, the spores germinate and resume growth as normal bacterial cells. In contrast, many viruses last only a few days, if that, outside their animal or human hosts. However, infections due to these agents may persist among the local population, and this too may prove a threat to an invading army.

Storage of the Agent

An infectious agent that persists may be inconvenient from the perspective of a military occupation. However, an infectious agent that decays very rapidly has the opposite problem—it is difficult to store.

Preparation of the Agent

Some pathogenic microorganisms are relatively easy to grow in culture, whereas others are extremely difficult and/or expensive to manufacture in sizeable quantities. One major drawback with viruses is that they can grow only inside host cells. Culturing animal cells is far more difficult than growing bacteria and the yields are much lower. Large-scale manufacture of viruses is expensive and difficult. Many bacteria (e.g., plague, tularemia, typhoid) are relatively easy to culture. In the years just after World War II, the British kept large-scale cultures of Yersinia pestis , the agent of plague, growing continuously in case of need. Doubtless other nations did much the same.

Another factor is the issue of formulation. That is, the disease agent must be prepared in a manner that facilitates storage and dispersal. Both bacterial cells and spores tend to clump together spontaneously. Consequently, they must be weaponized to disperse them effectively by aerosols and other delivery systems. This issue is highly technical, and the details are beyond the scope of this book.

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