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Infectious disease agents and how infection occurs

Infectious disease agents and how infection occurs
PROFESSOR RAINA MACINTYRE: For this course, it’s important to understand the basics of infectious diseases. While we can’t teach you everything there is to know about infectious diseases, we can cover the basic principles that you must understand in order to understand concepts of bioterrorism. Infectious diseases are a range of different clinical syndromes caused by different pathogens, which can infect people in different ways and spread by different routes. They can be caused by bacteria, viruses, fungi, parasites, or prions. In terms of the global burden of disease, although heart disease and ischemic vascular disease is at the top of the list of the cause of deaths globally, infectious diseases still occupy several places in the top 10 causes of death.
HIV, TB, and malaria alone account for more than 13 million deaths annually. And there are more TB deaths than ever. And infectious diseases cause 25% of all deaths and 63% of child deaths. Bacteria are a microscopic cellular organisms, which can reproduce. They’ve got chromosomes and ribosomes. They usually have a rigid cell wall. Some have a little tail, called a flagella. And they can be either shaped like a rod, as you see in this picture here, or a coccus, which is more round shaped. And they are defined by how they stain with the gramme stain.
They are either gramme negative, which means the colour doesn’t come up, or they are gramme positive, which means a purple colour comes up on the stain. Now, the important thing about bacteria which differentiates them from viruses is they are extracellular. So they do not reside inside of cells, but in the bloodstream, for example, or in tissues. They can cause disease either by direct invasion, by toxin production, or by spores as in the case of anthrax. And most bacteria can be treated by antibiotics. And they can be prevented, some of them, by vaccines. Examples of bacteria are given there. Viruses, on the other hand, are smaller than bacteria. The largest virus, such as pox viruses are larger than the smallest bacteria.
But in general, they are much smaller than bacteria. They have a protein coat and a core of genetic material, which can either be DNA or RNA. And these, as opposed to bacteria, intracellular viruses, require a living cell in which to live and replicate. And the pathogenicity of a virus is caused by invading human cells and using these cells to reproduce. There are some antiviral drugs available, such as for influenza. But most viruses have no treatment. There are, however, vaccines for many viruses. Examples of viruses are shown here.
You might have heard the term superbugs. And this describes bacteria which are resistant to most known antibiotics, which means that antibiotics do not have any effect, because the bacteria can evade them. They generally have a high death rate. MRSA refers to Multi-Resistant Staph aureus, Vancomycin-resistant enterococcus, extreme drug resistant tuberculosis, which is an emerging threat globally, and a relatively new threat called New Delhi Metallo-beta-lactamase, which causes resistance in a wide range of bacteria. So what is it that’s unique about infections? If you think about a disease like asthma or diabetes, that can’t be transmitted from one person to the next. However, infectious diseases can. They’ve got this unique capacity to be transmitted from human to human or from animal to human.
And humans exist in mutually exclusive states of being either susceptible to an infection, infected, or immune. And infectious diseases, because they spread, can cause epidemics. You can develop immunity to an infection, either from becoming infected or by having a vaccine, which simulates the natural infection process. Now, very key to understanding bioterrorism is understanding the modes of transmission. If you think about anthrax, for example, anthrax causes toxicity and pathogenicity to humans through the spores, which are inactive substances and they are a one hit attack. So if somebody posts out anthrax spores in an envelope, for example, the person who opens that envelope will get sick. But it will not spread from that person to other people.
So transmission is either direct or indirect. Direct means there’s immediate transfer of the agent. When I say agent, I’m talking about the bacteria or the virus, from the host or the reservoir to an appropriate portal of entry, such as through kissing, through intercourse, ingestion, airborne droplets, and so on. And then, there’s indirect transmission, which is through an intermediate vehicle, vector, or airborne travel from the host or the reservoir to the person who’s becoming infected. Examples of vehicles, include inanimate subjects such as food. So an agent, remember, a bacterial virus, may or may not multiply or develop in the vehicle before being introduced into the human host. I’ve shown you ice cream there.
Staph aureus poisoning is very common in dairy foods, such as ice cream. A vector, on the other hand, is a carrier, such as an insect, which may or may not multiply the agent and may be used as a mechanical spreader. Usually, it does multiply. Now, the routes of transmission can be either person to person, in the case of smallpox and influenza, this is what happens. And this is the most serious kind of spread and this kind that can cause a pandemic or an epidemic. Soil, water, or food, or vehicle to person. Examples include salmonella, anthrax. And these cause point-source outbreaks. It’s a one hit exposure. And after that, if the source is identified and removed, there’s no more transmission.
And then, you get vector, which is animal or insect-borne transmission to person. Examples include plague and tularemia. Now, in terms of person to person transmission, respiratory transmission is the one we worry about the most with bioterrorism, because once one person is infected they can transmit it through the air to other people. And therefore, it can spread very rapidly. There’s also faecal oral transmission, which is essentially contamination with faecal material. Someone may touch that and then touch their mouth or touch their food. And things like hepatitis A and rotavirus can be spread in this way. Fomital contact spread, which is direct contact where you touch an object that is contaminated. Ebola virus is known to spread in this way.
And then, you touch yourself and contaminate yourself. Sexual transmission, bloodborne transmission, often agents that are spread sexually are also bloodborne, such as HIV and hepatitis B. And then, vertical transmission, which is transmission from the mother to the baby. Horizontal transmission occasionally occurs, which is, for example, children playing football or playing rough sports, contact sports, have been documented to transmit hepatitis B to each other. Now, every organism has a dominant mode of transmission. But most organisms have more than one mode of transmission. So it is a fallacy to think that a particular virus or bacteria can only be spread by one way. You can always find other modes in which that is transmitted.
Now, vector to person transmission, insects, malaria, Dengue Yellow Fever, are examples of vector-borne or insect-borne infections. Animal to human examples include typhus, bovine TB, and plague. In terms of bioterrorism, it’s interesting to note that there are now insect drones, which are completely robotic insects. And obviously, these can be used to target delivery of an infectious agent with very precise coordinates. And then, there’s actual live insects, which can be radio controlled with a little radio controller on their back. And I’ve given you some links there that you can have a look at, if you’re interested to know more. Soil, water, or food to person, Clostridium tetani, which causes tetanus is a classic soil-borne infection.
Foodborne toxins, Staph aureus, Clostridium perfringens, and salmonella. And then, waterborne, cholera, cryptosporidiosis, polio and hepatitis A are examples.
In this video Professor Raina Maclntyre talks about what infectious diseases are, the types of pathogens that cause them, and ways in which people become infected.
After watching this video, discuss the answer to this question in the comments section below:
How do bacteria and viruses differ in the way they infect people?
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Biosecurity and Bioterrorism: Public Health Dimensions

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