History of Infectious Diseases

First post for the new semester! This will probably be my last semester of coursework (at least for a while), as I'm doing Honours next year. (For those of you who are visiting this blog from overseas, Honours is an optional final year of undergrad, where you do some original research.) This first lecture was pretty much history and background, which was kind of to be expected. Without further ado, let's get started!

Understand that infectious diseases have been around for a long time

Some of our earliest evidence of infection comes from fossil remains of dinosaurs. For example, some T. rex mandibles with abscesses have been found, as well as Stegosaurus tail spikes with osteomyelitis. There were probably many more diseases, but since we can only look at bones and things that can become fossilised, we will never know. Homo erectus fossils with osteomyelitis have also been found.

Understand the co-evolution of humans and pathogens

Pathogens have been around for a very long time. It's estimated that prokaryotes have been around for around 4 billion years! Around 2 billion years ago, archaea species engulfed proteobacteria in a process called endosymbiosis, producing the first eukaryotes. The evolution of viruses has been a lot more elusive. We don't know exactly how viruses developed, but we have three main hypotheses. In the progressive/escape hypothesis, viruses arose from genetic elements that gained the ability to move between cells. In the regressive/reduction hypothesis, viruses are thought to be remnants of larger, cellular organisms. Finally, in the virus-first hypothesis, viruses predate or co-evolved with their current hosts.

Human development, of course, did not get rid of pathogens. Instead, pathogens grew with us. We evolve to cope with pathogens, and they evolve to cope with us. Unfortunately, the dynamic can be quite one-sided, as humans reproduce- and therefore evolve- at a much slower rate than bacteria and other microbes. Furthermore, when humans do produce the right antibodies and immune cells for knocking out a certain bacteria, these antibodies and cells are not passed down genetically.

It's not all gloom and doom, however, as sometimes we can get genetic resistance to diseases. For example, people with sickle cell anaemia are less susceptible to malaria, whereas people with thalassemia (a haemoglobin disorder) might be less susceptible to Plasmodium vivax (though we still don't know this for sure). People who lack the Duffy antigen are also less susceptible to P. vivax, as this bacteria usually attaches to the Duffy antigen.

Give examples of evidence of ancient diseases

As mentioned above, diseases have been found in dinosaur fossils. There are, however, many more recent examples. The first written record of herbal remedies was dated at 3000BC in Mesopotamia (modern-day Iraq). Eber's payrus, which is a lengthy collection of medical texts found in Egypt, was dated at 1550BC. Egyptian mummies have also shown evidence of smallpox, and papyrus paintings depict diseases such as poliomyelitis. In short, when people tell you that "nobody got sick before x was invented," feel free to bang your head against the nearest wall. (Or don't. Especially if it's tiled. Head + tiles = lots of pain. I'm not going to say how I know that.)

Moving on to 400BC, Hippocrates proposed that diseases were caused by miasmas, or "bad air." We know today that the miasma theory isn't true. However, Hippocrates did also make associations between climate, diet, and living conditions, which are still factors that we take into consideration with regards to diseases.

Name (A) several important people in microbiology history and (B) describe their important discoveries

• Fracastoro- An Italian physician in the 1500s who was probably one of the first to discuss something resembling germ theory. He suggested that diseases could be spread via direct contact, fomites (objects capable of carrying infectious organisms), or airborne contagion.
• Leeuwenhoek- Made microscopes to examine bacteria in 1683.
• Edward Jenner- You've probably heard of this guy before. He's the one who developed smallpox vaccine from the cowpox virus. This method was much better than earlier inoculation methods used in India and China in the 17th century: these methods used powdered scabs from people infected with smallpox, which was much riskier and carried a ~2% mortality rate.
• John Snow- The father of epidemiology. He monitored the spread of cholera in order to determine its source in the community (a particular pump that drew sewage-polluted water from the Thames).
• Pasteur- Debunked the "spontaneous generation" theory (theory that living things could spring from non-living things), developed pasteurisation to kill microorganisms in milk, and developed anthrax and rabies vaccines.
• Koch- Cultured bacteria on media, developed nutrient agar and broth, identified the causes of TB and cholera, and developed Koch's postulates (criteria for determining whether or not a particular pathogen causes a particular disease). Koch's postulates are as follows: 1) the microorganism must be present in sick organisms but not in healthy organisms, 2) the microorganism must be isolated and grown in culture, 3) the cultured microorganism should cause disease if injected into a healthy organism, and 4) the microorganism must be able to be re-isolated from the inoculated, diseased organism.
• Ivanowski and Beijerinck- Discovered infectious agents that could pass through filters. These infectious agents turned out to be viruses.