In recent years genetic diseases have become better understood since they are dependent on a fault in the normal gene sequence that controls body activities.
Cocci, in the chain shape, simple rod bacillus with its flagella; cocci in a cubicoidal arrangement; spirochite with its filament; vibrio with a stalk; and bacterium with numerous appendages (artwork by Claire Tremblay).


The word "disease" literally means uneasiness or the absence of ease and was first used in this sense. It has grown from meaning a sense of discomfort to a more specific medical condition. In general, disease can be defined as an unhealthy condition resulting in an incorrect function in an individual. This can affect one or more organs, a specific part of the body or the whole body. It can be mental or physical and extends to all forms of life, including bacteria, plants, animals and humans.

Genetic Diseases

In recent years genetic diseases have become better understood since they are dependent on a fault in the normal gene sequence that controls body activities. Thus, Down syndrome, which is characterized by a distinctive physical appearance and mental condition, is caused by a critical portion of chromosome 21 occurring 3 times instead of twice in some or all cells. In other diseases chromosomes are duplicated, deleted, inverted or translocated.

These GENETIC DISEASES are congenital, ie, the individual is born with the condition or with the propensity for the defect, as in Huntington's chorea, which usually does not manifest itself until an individual is 30 years of age or older. Other genetic diseases include club feet, heart defects and diabetes. One error in a DNA base sequence can prevent the production of an enzyme or hormone and cause biochemical imbalances that may result in death if untreated. Fortunately, many of these conditions today can be reversed, but they cannot be cured through dietary supplement or regular injections of the missing compound.

Nutritional Deficiencies and Unfavourable Environment

Nutritional deficiencies make infections more likely, especially as unhygienic conditions are often present where there is lack of proper food. Although this is more of a global problem, even in Canada some marginalized persons are in a state of chronic poor nutrition and are prone to infections.


Respiratory, enteric and sexually transmitted diseases are those we most frequently associate with the word disease, because human bodies have been invaded by bacteria, viruses or parasites. Vaccines have been produced to immunize children, and sometimes other populations at risk, from diseases that used to kill millions of people, such as scarlet fever, measles, diphtheria and SMALLPOX. Some infections, however, have proven difficult to control, including INFLUENZA, the common cold and SEXUALLY TRANSMITTED DISEASE (see AIDS), food-borne disease and WATER-BORNE DISEASE like salmonellosis and giardiasis (see SALMONELLA; HAMBURGER DISEASE; BOTULISM).

Vector-borne diseases are those that are transmitted to humans via an intermediary host such as insects or rodents. Canadians are more likely to be exposed to these diseases abroad but eastern and western equine encephalitis through the mosquito and Lyme disease through ticks are reported in this country (see MALARIA).

Since 2001, the mosquito-borne WEST NILE VIRUS has become a matter of concern in Canada. The virus first appeared in Ontario that year and since then has spread through the Maritimes and westward across the country. It first appeared in birds and animals, but has spread to humans. By the summer of 2005 there had been nearly 1400 cases reported among humans.

Diseases Caused by Toxins

Naturally occurring toxins or chemicals in foods or drinks (see FOOD POISONING ) can cause disease. Natural toxins that affect Canadians include botulinum toxin, causing botulism (from spoiled bottled vegetables and improperly preserved meat), seafood toxins such as paralytic shellfish poison (from shellfish in BC, the Gulf of St. Lawrence and the Bay of Fundy) and ciguatoxin when tropical fish are eaten (in the Caribbean and fish imported to Canada). Mycotoxins from mold growth on crops may occasionally affect domestic animals.

Chemicals in foods or the environment, even at low levels, are considered to be hazards, as they may be responsible for CANCERS and birth defects, although proof is difficult to establish for most of these because of the presumed long exposure period required. Tobacco and improperly used alcohol and drugs are also generally accepted to be hazardous substances (see ALCOHOLISM; NONMEDICAL USE OF DRUGS; SMOKING ). Some chemicals are toxic through skin exposure, as vapours and radioactive substances. Those likely to be exposed to these, eg, laboratory technicians, industrial workers, firemen and farmers, should be protected through appropriate outer clothing and masks.

The Body's Defences Against Infectious or Toxigenic Agents

The body has defences against invading agents through the immune system, which detoxifies, captures, and destroys or expels these agents. Infectious micro-organisms have an external surface (called antigens) that stimulates the production of antibodies once they are exposed to the immune system. Most toxins produced by microbes are also antigenic. These antibodies may remain in circulation for a lifetime, and therefore it is possible to immunize children against certain bacteria or viruses to protect them indefinitely. Those with an underdeveloped or weakened immune system are more likely to be infected. These include young children, aged or ill persons, people who have had organ transplants (see TRANSPLANTATION), people on drug therapy, and pregnant women.

Emerging Diseases

Public health officials once were satisfied that infections were a problem of the past and that other issues should be the focus of medical attention. However, many existing diseases are resurging and new diseases are emerging in many places throughout the world. Those affecting Canadians include AIDS; pulmonary hantavirus infections originating from deer mice; peptic ulcers from Helicobacter pylori; Giardia and Cryptosporidium parasitic infections from unfiltered water; and Severe Acute Respiratory Syndrome (SARS).

Factors cited for this emergence are population growth and migration, urban decay, reduction in public health control programs, deforestation, changes in water ecosystems, global warming, globalization of food supplies, aquaculture, increased immunosuppression through drugs and organ transplants, antibiotic-resistant strains of bacteria, microbial adaptation to change, increased travel, outdoor recreation, unsafe sexual practices and illicit drug use. Without a massive worldwide campaign to address these issues, infectious disease will play an increasingly important part in the socioeconomic environment of Canada and other countries.

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