Definition of disease A disease is an abnormal condition of an organism that impairs bodily functions, associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease (pathogenic micro-organisms), diet, stress and autoimmune diseases.

Regulatory effect of disease on a population In the absence of environmental resistance a population will increase until disease balances the growth rate. Compared with a sparse population, a dense population is more prone to wide- spread attack by a disease causing organism and is more likely to suffer high losses if the disease is fatal.

Bubonic Plague The bubonic plague (“the black death”) had a regulatory effect on human population throughout Europe during the 13th – 19th century. The incidence of bubonic plague outbreaks now is at a very low level. Other diseases which are more prevalent today exert a regulatory effect on human population, like HIV/AIDS which at present has no cure. AIDS kills over 3million people each year.

Factors leading to a drop in rate of disease Vaccinations Improved hygiene, sanitation and living conditions

Use of Vaccines Immunisation is the process by which a person’s ability to resist a disease-causing organism is increased Vaccination is a method of immunisation which deliberately introduces a weakened/altered form of the pathogen into the body in order to stimulate it to produce the appropriate antibodies The first vaccinations were against smallpox – this has since resulted in complete eradication of the disease

Why was the Small Pox Vaccine was Successful? Only humans were affected by the virus. There is no animal carrier. The immunity resulting from the vaccination persists for a very long time. Infectivity does not precede overt symptoms so an infected person can be identified and isolated. The virus acts a single antigen, making the production of a vaccine a relatively straight- forward procedure. There was a major commitment by the World Health Organisation and governments.

Problems with Vaccines Different types of virus (including mutations) may cause similar diseases. e.g. the common cold. As a result, a single vaccine will not be possible against such a disease. Large animal reservoirs. If these occur, re- infection after elimination from the human population may occur. e.g. Malaria Transmission from cell to cell (via syncytia) - This is a problem for potential AIDS vaccines since the virus may spread from cell to cell without the virus entering the circulation.

Living Conditions Developed Countries: High standards of hygiene, waste disposal and nutrition all contribute, directly or indirectly, to the reduction of disease.

Living Conditions Developing Countries: Greater than 50% of population in such countries do not have access to clean drinking water or sewage disposal facilities. Sewage contaminated water contains pathogens which can cause several diseases.

Control of Childhood Diseases Developed Countries: Prevalence of diseases such as measles, whooping cough, diphtheria, tetanus, polio and tuberculosis are successfully under control. Higher standards of living (e.g. Clean water in abundance, hygiene and nutrition) and vaccinations have ensured this.

Control of Childhood Diseases Developing Countries: A lack of information, understanding and financial means often results in parents failing to bring children to health clinic for vaccination. Of those that are brought in for the first dose, many fail to return for the second or third doses required for full protection. This means the majority of these children are left susceptible and die of diseases which are now rare in developed countries.