This is the chapter 13-14 video. We're going to talk about disease, disease transmission, microbial mechanisms of pathogenesis, and epidemiology. So what is disease?
The terms infection and disease are often used interchangeably, but infection is when a contaminating organism evades the body's external defenses and multiplies in the host. A disease refers to an infection that would get out of control, basically. It results when normal body functions are altered. So with bacteria and viruses, these cause what are called infectious disease, so diseases that can be transmitted to one another. And when you are diseased or have an infection, there are usually signs and symptoms of disease.
So symptoms are subjective characteristics of disease felt only by the patient, such as, you know, pain. Signs would be... objective manifestations of disease observed or measured by others, so fever.
Some diseases, you may actually be asymptomatic, so that you would actually lack symptoms but still have signs potentially, and the disease can be transmitted. The stages of infectious disease are, there's basically five stages. We have the incubation period, which is the time between the infection and first symptoms and signs of disease.
Prodromal period, which is usually a short amount of time with mild symptoms. The illness phase, which is the most severe stage where signs and symptoms are most evident. A period of decline, which is a gradual decline of signs and symptoms as the body returns to normal.
Convalescence period, when the patient recovers with no signs or symptoms. Now depending on the infectious agent, the patient can be infectious at any time, at any of these stages. So it depends on the actual cause of aging. These stages of infection can correlate to the number of microbes in the system.
So when you are exposed to the antigen or the infectious agent, you go through that period of having no signs or symptoms, but there is the microbe present at a low level. And as the... stages progress, the number of microbes in the body would increase. There are a few different ways we can classify infectious diseases. There's acute disease, which is a rapid onset, short period of time, so something like the common cold.
Chronic disease, which develops slowly, or may be actually recurrent, such as hepatitis or tuberculosis. And then latent disease, where the pathogen remains inactive for a long period of time before. coming active again.
So example there would be shingles. So how do we actually acquire a disease causing organism? We have to come into contact with a reservoir and infection. So these are where the site of the site where the pathogen is maintained. So it's able to survive in that reservoir.
And the reservoir may or may not be harmed by the pathogen. There are three types of reservoirs. There's human reservoirs.
And since we are human, this is the principle. living reservoir for us humans. So people with signs and symptoms may transmit disease or individuals who are asymptomatic may transmit disease. You know also you could be a carrier which is somebody which is somebody who's considered to be asymptomatic and infected for years potentially months or years going unchecked in that person's immune system may just keep that organism in check. for a long period of time, but those carriers can still transmit the disease.
There's animal reservoirs. Animal diseases are referred to as zoonosis or zoonotic diseases, and there's approximately 150 different zoonotic diseases. And these diseases are spread naturally from animal to the host, human host, through various routes, either direct contact with the animal or its waste.
animal bites or insect bites. Non-living reservoirs such as soil, water, food can be reservoirs as well. The presence of the microorganism is often due to contamination by feces or urine or can be naturally found in the environment such as tetanus and botulism.
Water, for example, would be a source of potentially parasitic eggs such as giardia or E. coli from fecal contamination. And then your meats and vegetables other food products contaminated with E.
coli or salmonella again from the cross-contamination of feces or urine. Many pathogens cannot survive or remain active outside of a host for very long, and a successful pathogen needs to overcome the innate and adaptive immune responses long enough for the pathogen to colonize and exit the host. So it's usually it's just a disadvantage for the pathogen to actually kill the host, since the opportunity to be transmitted would be eventually limited. And this phenomena of basically reduced pathogenicity over time is referred to as balanced pathogenicity.
So an organism becoming less harmful over time, more or less. Once you come into contact with a reservoir of infection, that organism has to go through a portal of entry. These are sites which the pathogen enters the body and there's four major pathways. There's the skin, which the outer layer of skin is dead and acts as a physical barrier to pathogens. So if you're going to enter through the skin, they're going to usually enter through an opening such as a cut or abrasion.
Others, which is less common, can burrow or digest the outer layers of skin such as the little scabies mite here. Mucous membranes, these line the body's cavities that are open to the environment, so the respiratory tract is a common entry. The GI tract may be a route of entry as well, if the organism can survive the acidic pH of the stomach. Placenta is another portal of entry. The placenta typically forms an effective barrier.
There are certain pathogens that can cross the placenta, such as listeria, and can cause... spontaneous abortion, birth defects, premature birth. So that's one of the reasons why pregnant women are encouraged not to consume unpasteurized foods such as cheeses and lunch meats in order to reduce the risk of contracting listeria basically to zero.
And then there's the parenteral route, which is not a true portal of entry, but it's a way Portals and entries can be bypassed, so it's a mechanical depositing of pathogens directly beneath the tissue. So hypodermic needles or thorns or nails, things like that, piercing the skin and depositing those organisms beneath the skin. There are different modes of infectious disease transmission from the reservoir through a portal of exit to another person's. portal of entry. So there's three groups of transmission.
There's contact, vehicle, and then vector transmission. So contact transmission, there's direct contact where there's physical touching, handshaking, sexual intercourse, the fecal-oral route would be methods of direct contact between two people transmitting the disease-causing organism. Indirect contact, which is a transfer of pathogens.
through an inanimate object referred to as a fomite. So this would be clothing, doorknobs, glasses, your lab benches, and things like that. Another contact transmission would be droplet.
This is the transmission of a pathogen through the inhalation of respiratory droplets. So close proximity is usually necessary. This would be the transmission through coughing and sneezing and inhalation of little droplet nuclei.
The large infectious droplets usually fall 1 to 3 feet, smaller ones 3 to 5 feet, but the small little nuclei can remain in the air for longer periods of time or actually go further if when you do sneeze for example. Vehicle transmission would be airborne, so the pathogen is suspended in the air and travels in the air to another host. And these can be very difficult to control because, well, you can walk into a room after somebody sneezed hours later, an example of measles, and potentially if inadequate filtration and things like that could spread those pathogens to other rooms. Another vehicle of transmission would be food. Usually these are digestive tract diseases, and contamination can occur through a variety of different ways.
The animal can harbor the pathogen, your E. coli and salmonella. or inadvertently added during the food preparation, so either hand washing with Staphylococcus aureus, cross-contamination with cutting boards, meats, and vegetables, or improper butchering processing practices. Foodborne pathogens can cause one of two different types of infection, one referred to as foodborne intoxication.
which is the ingestion of an exotoxin produced by an organism growing on the food product. So the toxin is actually causing the illness, not the living organism. The organism is killed by heat, usually, but the toxin may or may not be heat-stable.
So in the case of Staphylococcus aureus, it actually produces a heat-stable toxin, so it can survive a cooking process. The Clostridium botulinum toxin is inactivated through heating. process. And then there's foodborne infection, which requires the consumption of living organisms, such as undercooked hamburger or chicken, for example.
And usually with a foodborne infection, the symptoms don't usually appear for at least a day, most notably causing diarrhea and vomiting. Thorough cooking before consumption usually would kill those organisms. So E. coli, salmonella are easily killed by properly heating. the food to 160 degrees Fahrenheit.
And then vehicle transmission with water. This can involve large numbers of people. So for example, in 1993, this organism called Cryptosporidium parvum, an intestinal parasite, which causes watery diarrhea with a very low infectious dose of less than 10 organisms in Milwaukee.
infected 400,000 people from a municipal source. So even though if you get municipal water, it doesn't mean that it's always 100% safe. And then vector transmission, there's biological vectors and mechanical vectors.
Biological vector is when an organism transmits and serves as a host. So the vector is involved in the life cycle of the pathogen. So malaria, for example, part of the life cycle of the Plasmodium occurs inside of the Anopheles mosquito. The mechanical vector would be when the vector transmits disease-causing organisms through mechanical contact.
So chlamydia trachomatis carried on the feet of a fly from an infected person's eye to a new host. So it's a mechanical depositing of the organism, and the organism is not part of the life cycle of the vector. So, um...
We have to have movement of the pathogen into the host through a portal of entry, and then out of a reservoir through a portal of exit, if it's a human source. So the pathogen would leave the host through a portal of exit, which is usually the same as a portal of entry, or often the same. The pathogen would usually leave the host in the materials the body secretes.
excretes. And the human activities promote the transmission of such organisms. So these would be the different portals of exits, so broken skin and blood, for example, urine, feces, the other listed exits here. This concludes part one of the chapter 13 video.