Pathogens (SL IB Biology)

• A disease is an illness or disorder of the body or mind that leads to poor health
• Each disease is associated with a set of signs and symptoms
• A pathogen is any microorganism that causes disease in another organism (e.g. in plants or animals)
• Many microorganisms are pathogens including:
  • Bacteria
  • Fungi
  • Protists
  • Viruses
• Not all species within these groups (apart from the viruses) are pathogens, as many bacteria, fungi and protists are harmless and do not cause disease
• However, all viruses are pathogenic as they can only exist by living inside the living cells of other organisms (or by using these cells to create more viruses)
• No archaea are known to be pathogenic in humans
• Pathogens cause communicable diseases which means they transfer from a diseased host to a healthy organism during infection, in other words the disease is infectious
• Examples of such diseases include:
  • tuberculosis
  • athletes foot
  • malaria
  • cholera
• Non-communicable diseases are non infectious diseases such as
  • cancer
  • cardiovascular disease
  • malnutrition

Infectious & Non-infectious Diseases Table

NOS: Careful observation can lead to important progress

• Observations have lead to many medical breakthroughs in the treatment of disease
• These observations have allowed a deeper understanding of diseases and the pathogens that cause them
• Knowledge of symptoms, incubation times and transmission mechanisms are all important in order to implement measures to control the spread of a disease
• The optimum scenario is to eradicate the disease completely

Cholera

• Cholera is a water and food-borne disease caused by the bacterium Vibrio cholerae
• Cholera can be transmitted when people are exposed to contaminated water, either through consumption or through bathing
  • The disease is common where people do not have access to proper sanitation (clean water supply) and uncontaminated food
  • Infected people pass large numbers of the bacteria in their faeces 
  • If these faeces contaminate the water supply (due to lack of proper sewage treatment), or if infected people handle food or cooking utensils without washing their hands, then the bacteria are transmitted to uninfected people
• In 1854 a Cholera outbreak in Soho in London lead to the death of over 500 people in a month
• A local doctor, John Snow, observed the clinical presentation of the disease after encountering an outbreak in a mining village in 1832, and so was familiar with the symptoms and mechanisms for transmission
• His prior experience provided a fundamental insight which helped him to identify the cause of the outbreak in Soho
• He mapped the cases of cholera and traced them all back to one water pump
• The pump handle was removed and the outbreak came to an end
• Later it was noted that the water pump was positioned only a few feet from a cesspit which was contaminated with Vibrio cholerae
• John Snow's careful observations facilitated the control of this spread of cholera in this situation and provided useful evidence which became incorporated into the 'germ' theory of disease which revolutionised sanitation in the 19th century

Childbed fever

• Puerperal fever, also known as childbed fever, is a bacterial infection of the female reproductive tract after childbirth
• Transmission of the disease occurs through direct contact during the delivery process
• Childbed fever was the most common cause of death associated with childbirth in the 19th century
• A Hungarian physician, Ignaz Semmelweis, observed that there were a greater number of deaths in one maternity ward compared to another
• On closer investigation, Semmelweis noted that obstetricians and medical students who took part in autopsies (of women who had died from childbed fever) went on to deliver babies in the maternity ward without washing their hands
• He found a correlation with the number of deaths in the ward and the number of autopsies carried out leading him to suggest a link between handling the corpses and the number of new cases
• Semmelweis suggested that particles were being transferred from the corpses to the women on the maternity ward
• He initiated a mandatory hand washing policy for all those involved and later also began washing the medical instruments
• These precautions lead to a clear decline in patient deaths from childbed fever and informed the foundations of routine hand washing routines in healthcare
  • Such processes are fundamental, particularly in hospitals, to the control of many transmissible diseases