Introduction – Jan. 7
- Infectious disease is the major cause of human mortality throughout history. Major increases in life
expectancy from the early 1900s onward were mainly due to control of infectious diseases. Life
expectancy is highest in North America, Europe, and Australia. Of the top 10 causes of death in high-
income countries, only one is from infectious disease (lower respiratory infections). In low-income
countries, lower respiratory infections, diarrhoeal diseases, HIV/AIDS, malaria, TB, and neonatal
infections are all infectious diseases found in the top 10 causes of death.
- Do infectious agents evolve to become more virulent?: There are multiple possible outcomes when a
population is infected by a new infectious agent – 1. The host’s immune system overwhelms the
agent. 2. The agent overwhelms and kills the host without being transmitted. 3. The agent replicates
within the host and is transmitted to a new host. The host may die or recover. New infections may
produce a high death rate, but over the course of time, the host and agent adapt to one another and
the disease normally becomes less virulent.
- Epidemiology of infectious disease: Endemic – When an infectious disease is continuously present.
There is a continuous supply of new hosts. In smaller populations a disease cannot find sufficient
new hosts. Epidemic – When new cases of a disease substantially exceed what is expected.
Populations lack sufficient immunity to an infectious agent. Pandemic – An epidemic on an inter-
country or inter-continental scale. HIV is an epidemic because it is still increasing.
- Human-microbe relationships: Parasitic – Relationship between two species in which one benefits
(parasite) from the other (host). It usually involves some detriment to the host. Commensal –
Relationship between two species in which one is benefited and the other is not affected, neither
negatively nor positively. Mutualistic – Relationship between two species where both benefit.
- The microbiota: Modern medicine has studied microbes as perpetrators of disease. However, the vast
the majority of microorganisms are not pathogenic. In humans, there are - resident
microorganisms and they are termed the microbiota. They are mostly concentrated in the
gastrointestinal tract. There are more than 1000 different species and most have not been cultivated.
We have more bacteria in our bodies than human cells.
- The human microbiota: Internal organs are usually sterile. Surface tissues (external) have extensive
populations of microbes. The collective genome of the human microbiota (the microbiome) contains
more than 100 times as many genes as our own genome. Som