In this lecture on infectious diseases, I’ll be covering a number of topics in the various chapters. This first one will be on the changing patterns of infectious diseases in time and place, that is looking at infectious diseases in a somewhat historical context.
I’ll also be covering the control or eradication and elimination of infectious diseases, zoonoses, emerging and reemerging infections, and lastly diarrhea and respiratory diseases in children as they are a major killer in this group of individuals.
Before we get into this historical look I want to give you some common definitions. What is an infectious disease? It’s a disease caused by the entrance into and growth and multiplication in the body of…of an individual of bacteria, Rickettsia, viruses, parasites, protozoans, fungi and prions. All of these are considered infectious agents. Another term which we should become familiar with is incidence. This is the number of new cases that occur per unit time. That time could be a day, a week, a month, a year. But time is very much a factor here. Prevalence on the other hand is the number of cases that occur at any one moment in time, today for example.
I’m going to give you an extremely abbreviated history of epidemics, that is a large number of cases occurring, although that number could be just one. And pandemics which are epidemics that cover the global world. Diseases associated with many of these epidemics include plague, smallpox, influenza, cholera, yello fever and measles. Now some of these are more historical because they don’t occur today, especially smallpox and plague and yellow fever, but they are important. An example of a devastating pandemic was the black death from plague that peaked in the 1346 to 1350 time period and it was estimated to have killed 30% to 70% of European population. Smallpox and tuberculosis, killed up to 90% of the indigenous population of the New World when they were introduced by the Europeans. Cholera pandemics began in 1816 and now take up, we’re in the seventh pandemic worldwide.
The largest single pandemic was that of the 1918 influenza pandemic which led to between 75 million plus deaths worldwide. These infectious diseases can be noted in history where they left their mark. For example in this mummy Ramesses V dated about 1157 BC we can see evidence of smallpox lesions. The pandemic epidemic that I want to focus on in the next pictures are those of plague. Plague was so common and was such a killer in Europe that there were dances of death that were developed to deal with the large numbers who were dying in various communities. This is a picture of a bubonic plague from one of the Bibles and we would probably not find these lesions to be consistent with plague today but this is the way people saw them. Doctors would oftentimes have very little to offer and would go around in these long black coats wearing these masks shaped like that of a bird’s head which limited the air that they would breathe in since they believed,in some cases correctly, that the miasma- the surrounding air, is what caused these diseases to occur in populations. People were so afraid and felt that the devil had entered their body and that’s why they would get these diseases that people would go around the countryside flagellating themselves to try to get rid of these evil forces and to show that they were deeply religious. Unfortunately, one of the things that often occurs in any epidemic or pandemic is we tend to blame the most impoverished, the poorest people who are there as we see them as the ones that have caused this to happen. And of course this is oftentimes very untrue. Jews who were burned alive because they were thought to be the cause of the Black Plague.
Oftentimes people were labeled as witches, as other non-desirables in populations and so that if we got rid of them, the plague would go away. Of course we even do this today and we’ve done it today with HIV/AIDS and other similar types of pandemics.
This is the organism that causes plague. This is called Yersinia pestis, it’s shaped a bit like a safety pin and these are transmitted through these fleas. Obviously not this large
but these fleas live on the body of rats, Rattus rattus and so oftentimes when there would be a rat die off these fleas would escape and then bite humans causing the plague. Sometimes there were large numbers of rats because the cats had been killed in some communities because they were seen as the animals of the devil. In addition to the tremendous number of deaths that occur, we also have to look at the historical changes brought about by any large pandemic. For example in the Black Death there were huge demographic changes. With 30% to 70% mortality in many areas of Europe, which took at least 150 to 250 years to recover. There were labor shortages, serfdom disappeared, wages became higher, labor saving technologies developed because there was a shortage of labor, land became plentiful, see these were all actually positive outcomes, despite the high numbers of death. However religious intolerance and the tradition of blaming others also increased and this was a time, oftentimes of religious intolerance that went through the countryside. Let’s go back to the terminology that we started talking about. If one is exposed to an agent, an infectious agent, a number of possibilities could occur. That is nothing, there could be clinical infection, the second line, or sub clinical infection or the person could carry the organism but not be sick. For clinical infection, one could see that death might occur, immunity that is what that an individual recovering from the disease might be immune or they might carry it or they might be non-immune and for the sub clinical infection of course there wouldn’t be death but immunity, carriage, and not-immunity might also occur. Also there are a number of factors that influence disease transmission and we should be aware of these. One is the agent itself which is the bacteria, viruses and so on. How infectious is it? What kind of illness does it cause?
What leads to its survival or death? Then there’s the host which is humans or animals, our age, our sex, our behavior, our nutritional status and so on, our health status, and then there’s the environment in which we live. What is the weather like? The housing? How crowded are people? What is the geography and what occupations do we have at the time that make it more susceptible. What’s the quality of the air, the food, our socioeconomic status and even the political nature of the situation. Now again, when something infects an individual there are a number of outcomes. An individual becomes infected and then there’s an incubation period which then leads to clinical disease or no disease. When somebody is incubating that infection, that is they’re not sick but that it’s growing in their body, there is a latent period. A period when they’re not infectious at all. They can become infectious, even before they get clinically ill, and it is during that infectious period that they can infect another. So you see in the first patient there’s the incubation and clinical disease, a latent period, in infectious period, and during that infectious period they infect the second patient.
This gives you an example of how infectious diseases have changed historically. In 1909 the percentages of infections in Chile causing mortality and morbidity was about 50%. Fast forward to 1999, those figures are now down to about 22%…23% so in 99 years, or in 90 years, we have seen a dramatic reduction in the importance of…of infections as causes of mortality and morbidity. We’re going to look at what we can do to reduce the importance of infectious diseases. There are a number of basis of infectious disease control. One, what is the quality of water and sanitation? What kind of water are we putting into our body? And how do we dispose of human waste and other waste? What about improved hygiene, hand washing for example. Housing- By decreasing crowding we can decrease diseases like tuberculosis. Improved nutrition also has a bearing because the host is stronger and able to combat certain infections. In the last 70 years or so we have developed incredible array of vaccines and antibiotics or antimicrobials. And lastly there have been certain behavioral changes. Here’s a young girl in a program that is teaching kids the importance of hand washing and how you can use soap and water to reduce risk, particularly after…after going to the bathroom for example. Unfortunately in crowded cities in many low income countries waste disposal through these latrines which are over-hanging a body of water in which kids sometimes swim and which are right across from a better off section of a city also leads to environmental contamination and increases the risk of the transmission of infectious agents. Far better that if we could put even this simplified form of a SanPlat latrine which people could use to get rid of waste rather than putting it directly into the water as shown earlier. Going back to vaccine development, the first one was in 1796 where Jenner used cowpox to in fact protect people against smallpox. We’ll see more of this later. In the 1800s we then had the development of rabies, cholera, typhoid and plague vaccine, but the huge increase occurred in the 1900s with diphtheria, pertussis, BCG which is for tuberculosis. In the 1950s we developed poliomyelitis vaccine and then in the 18…1980s, ’90s and to the present day there’s been a huge development in vaccines such as hepatitis A and B, pneumococcal rotavirus, meningitis, et cetera. The first antibiotics interestingly enough, this was not the first but one of the first, go back really to the late 1930s with sulfonamides but here is streptomycin, 1943 it was developed by Selman Waksman and first given to patients in 1944 for tuberculosis and you can see from this list of tuberculosis drugs that at least seven, and there are many more actually that have been developed since that first…since the first discovery of streptomycin. We now have other anti tuberculosis drugs which are used particularly against the drug resistant tuberculosis strains. Now lastly, I want to go back to our examination of the basis of infectious disease control and indicate a few other measures that we can use. One of these is surveillance and reporting, that is who is getting the disease, where are they getting it, and so on. So that we can then institute certain disease control programs, be it against malaria, other vectors, parasites. And we can institute preventive measures such as the promotion of hand washing, screening, education in schools, behavior modification, social marketing and so on and lastly the development of early and effective and affordable therapy to deal with the infections that are…have occurred in the past and will continue to occur into the future. So in this first brief lecture, I’ve tried to give you a few definitions of infectious diseases and how they might be transmitted. I’ve tried to look at the history of one of these, that is bubonic plague and pneumonic plague, and lastly to give you a brief outline of some of the basis for infectious disease control.