Characteristics of Infectious Disease: Study with Video Lessons, Practice Problems & Examples

Infectious diseases arise from the colonization of pathogens, leading to observable signs like fever and subjective symptoms such as pain. Communicable diseases spread between hosts, while noncommunicable diseases, like cancer, do not. Primary infections weaken the immune system, making individuals susceptible to secondary infections from opportunistic pathogens. Virulence indicates an organism's ability to cause disease, with virulence factors enhancing pathogenicity. Understanding these concepts is crucial for recognizing disease mechanisms and improving public health strategies.

Intro to Characteristics of Infectious Disease

In this video, we're going to begin our lesson on the characteristics of infectious disease. And so it's appropriate to first start off with the definition of disease. Disease can be defined as a condition that is not the result of a physical injury where the normal structure and function of a host is either damaged or impaired. The term infection can be defined as the colonization and the replication of a pathogen or a disease-causing agent within a host. If we take these two terms, infection and disease, and put them together, we can define infectious disease as a spreadable disease that is caused by an infection that results in the patient showing signs and symptoms. It's important to be able to differentiate between signs and symptoms. Signs can be defined as objective evidence of infection that can be directly observed, measured, or detected by another person other than the person with the infectious disease. Examples of signs include a rash, swelling, and fever, which again are all signs of infection because they can be directly observed, measured, or detected by another person other than the person experiencing the infectious disease. Now symptoms, on the other hand, are more so defined as subjective evidence of infection that can really only be experienced by the person with the infectious disease. These symptoms cannot really be directly observed, measured, or detected by another person. They're only felt by the person experiencing the infectious disease. Examples of symptoms include pain and nausea, which can easily be detected by the person with the infectious disease but cannot be detected, observed, or measured by another person. Some diseases are also referred to as asymptomatic diseases. Asymptomatic diseases are diseases that are caused by an infection, but they result in the patient showing absolutely no signs or symptoms of the disease. These people that have asymptomatic diseases may not feel sick at all. However, they can still contain the infectious agent which can be colonized and replicating within that person, and so they can still be a source of infection for other people. If we take a look at our image down below, we can get a better understanding of the difference between signs and symptoms as well as asymptomatic disease. Notice over here on the left-hand side, we're showing you a person that has quite a high fever. Because fever can be measured by temperature and detected by another person just by using some kind of thermometer, a person with a fever is showing a sign, a detectable sign of infection. Again, signs can be directly observed and measured by another person other than the person with the infectious disease. Now on the right-hand side over here, we're showing you a person that is experiencing pain and is feeling nauseous. This person that is feeling nausea and pain, they are showing symptoms of infection, and that's because the pain and the nausea can really only be experienced by the patient with the infectious disease. But they cannot easily or directly be observed, measured, or detected. These are categorized as symptoms. Now over here on the right-hand side, we're showing you an example of an asymptomatic disease. Notice that this person here is saying that they feel fine, and they are not showing any signs or symptoms. However, notice that this particular infectious agent here, which seems to be replicating within their lungs, is still colonized and replicating within this person. And so they still have the disease, they are just asymptomatic with the disease. They can still be a source of infection and they can still cause other people to acquire the infection. Some diseases are or can be asymptomatic for some people. This here concludes our brief introduction to the characteristics of infectious disease. As we move forward in our course, we'll be able to learn more about them. So I'll see you all in our next video.

Communicable vs. Noncommunicable Diseases

In this video, we're going to differentiate between communicable diseases and noncommunicable diseases. Some diseases can actually easily spread from one person to another, while other diseases cannot spread from one person to another. Communicable diseases, which are also sometimes referred to as contagious diseases or infectious diseases, are diseases that can spread between hosts. These communicable, contagious, or infectious diseases are diseases that can easily spread from one person to another. Now, the term "infectious dose" refers to the specific number of microbes that are required to establish an infection in a host. A pathogen that has a very high infectious dose will only cause disease if the person is exposed to a very high number of microbes. Whereas a pathogen with a very low infectious dose can cause disease even if a person is only exposed to a very small number of microbes. We'll be able to get a better understanding of communicable, contagious, or infectious diseases as well as the infectious dose down below on the left-hand side of our image when we get there.

Noncommunicable diseases, on the other hand, are noninfectious diseases that do not spread from host to host. They do not spread between hosts or from one person to another. An example of a noncommunicable disease would be cancer, which is not communicable. It is a noncommunicable disease because cancer does not spread from one person to another. These noncommunicable diseases can arise from genetic, physiological, lifestyle, and environmental factors, and they're not typically the result of a pathogen. Communicable, contagious, or infectious diseases, on the other hand, typically result from pathogens or disease-causing agents.

We will take a look at the left-hand side of our image over here to get a better understanding of communicable diseases. Recall that these diseases can spread from one person to another. Notice we have an unhealthy person showing signs and symptoms and a healthy person. That infected person seems to be coughing and sneezing, spreading microbes. If this microbe has a very high infectious dose, then it takes a lot of microbes to make you sick. If this person is only exposed to a low number of microbes, then that would not be enough microbes to make this person sick, and so they would remain healthy. However, if this person is exposed to a high number of microbes, when the microbe has a high infectious dose, then that would be enough to make them sick. Notice here that they have a thermometer in their mouth and they're not feeling very well.

On the right-hand side, we're showing a noncommunicable disease that does not spread from one person to another. Notice that this person over here is smoking tobacco, which contains all types of carcinogens and mutagens, things that cause cancer and mutations. This person loves smoking, but smoking can lead to cancer. Cancer is an example of a disease that is noncommunicable because it does not spread from one person to another.

This here concludes our brief lesson on communicable versus noncommunicable diseases, and we'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

Which of the following would be considered a sign of a disease or infection?

• A. Headache.
• B. Pain.
• C. Nausea.
• D. Fever of 39°C (102°F).
• E. Lack of appetite.
• F. All of the following can be signs of disease or infection.

Diseases or infections that can be transmitted from one individuals to another are categorized as what types of diseases?

• A. Symptomatic.
• B. Clinical.
• C. Communicable.
• D. Noncommunicable.
• E. Latent.

Which of the following illnesses is an example of a noncommunicable disease?

• A. Infection with a respiratory virus like the flu or Covid-19.
• B. Food poisoning due to bacterial toxins found in canned food.
• C. Skin infection and blood infection acquired from a dog bite.
• D. Infection caused by an accidental puncture by a contaminated needle.

Determine if the diseases below are communicable or noncommunicable diseases.

A. Communicable Disease. B.  Noncommunicable Disease.
_____ 1) Lung Cancer.            _____ 3) HIV.                         _____  5) Diabetes.
_____ 2) Measles.                   _____ 4) Stroke.                    _____ 6) Covid-19.

Alright, everyone. Now that we know the difference between communicable and noncommunicable diseases, let's try to figure out which category these different diseases fall under. And if it is a communicable or transmissible disease, we're going to give it an 'a'. If it's noncommunicable or cannot be transmitted from person to person, we're going to give it a 'b'. Okay. So let's have a look. Number 1 says lung cancer. So remember that lung cancer, you probably met someone who has cancer. They cannot give you cancer. That is because cancer is caused by genetic abnormalities in your own body, in your own cells that cause tumors to be grown, but you cannot give that disease to somebody else. It's noncommunicable, so that means we're going to put a 'b' for number 1. Number 2 is measles. This is a highly contagious virus that most American children are going to be vaccinated against when they're small, in infancy. And this is part of the MMR, measles, mumps, rubella vaccine that most children commonly get because this is a communicable disease that's very contagious and easily transmitted. So this is an 'a'. Okay. Number 3, HIV. Human immunodeficiency virus is a bloodborne virus and it is also highly transmissible and communicable, and one individual can give it to another. Answer choice 4, stroke. There are many things that cause strokes, and some infectious diseases can lead to stroke, like COVID-19 can lead to stroke in some older individuals, but the stroke itself can also be caused by many other things: genetic factors, age, smoking, a whole bunch of different things. The stroke itself is not communicable. So we're going to put a 'b' here because you cannot give your stroke to someone else. It may be caused by a virus that you can give to someone else, but the stroke itself cannot be transmitted. Answer choice 5, diabetes. So this is going to be the body's difficulty in processing blood sugar and this is not communicable. A person with diabetes cannot give you diabetes. So this is going to be a 'b'. Alright. Finally, the big one, COVID-19. We should all know that this most certainly is communicable and contagious, so that is going to be an 'a'. Alright, everyone. Let's move on to our next video.

Pathogens that are very virulent are more likely to cause disease than pathogens that are less virulent (Virulence:the ability of a pathogen to cause and infection/disease). The infectious dose for Virus X is must lower than the infectious dose of Virus Y. Which virus is more virulent?

• A. Virus X.
• B. Virus Y.

Primary vs. Secondary Infections

In this video, we're going to differentiate between primary infections and secondary infections. What's important to note is that an infected person may actually be more susceptible to a second infection because of the damage done to their body by the first infection. Therefore, the initial infection can make a person weaker and more susceptible to a subsequent infection. Scientists refer to the initial infection as the primary infection, defined as the first or initial infection caused by a primary pathogen, such as the virus HIV or human immunodeficiency virus, which is a virus that attacks and kills human B cells or B lymphocytes that are part of adaptive immunity. Primary pathogens such as HIV are capable of infecting even healthy individuals that are not previously infected, and they will do this in a primary infection.

The secondary infection, on the other hand, is, of course, a second infection that occurs only after the primary infection. A secondary infection is caused by opportunistic pathogens, which usually do not cause disease under normal circumstances with a person who has a normal and healthy immune system. However, these opportunistic pathogens, as their name implies, are capable of taking advantage of infecting a compromised host that may be already infected with a primary infection. Thus, a person is more susceptible to a second infection by an opportunistic pathogen after they are exposed to a first or primary infection.

If we examine the image below, we can get a better understanding of this concept and how opportunistic pathogens can infect a compromised host. Notice on the left-hand side, we have a healthy person who does not have an infection, and we're going to assume that this person has a normal, healthy, and active immune system. Notice that their immune system cells, the B cells, and the T cells, are very strong and ready to fend off even opportunistic pathogens that may try to cause harm. However, on the right-hand side, we're showing you a person who has acquired a primary infection, such as a person that has the primary pathogen HIV, which can lead to AIDS, acquired immunodeficiency syndrome. This is a primary infection, and HIV is able to cause a primary infection even in a healthy person. You'll notice here that this person has a primary infection and that they have a weakened immune system, which makes them more susceptible to a secondary infection by opportunistic pathogens. Whereas before, these opportunistic pathogens could not cause harm or disease, now they are willing to take the opportunity to attack and cause harm when the individual is compromised by the first or primary infection.

This concludes our brief lesson on primary and secondary infections, and we'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

Opportunistic pathogens are least likely to infect which of the following groups of people?

• A. Cancer patients.
• B. Drug addicts.
• C. Organ transplant recipients.
• D. AIDS patients.
• E. High school students.

Virulence

In this video, we're going to discuss virulence. And so the term virulence can be defined as the degree of pathogenicity of an organism, or in other words, virulence is the degree of disease production of an organism, how well it can cause disease. And so an organism with very high pathogenicity is going to be more virulent, and that means, of course, that it is going to be more likely to be able to cause an infection and more likely to cause disease. Whereas, of course, an organism with a lower pathogenicity would be less virulent and would be less likely to cause an infection and less likely to cause disease.

Now the virulence factors can be defined as the specific traits that an organism has that allows it to cause disease, or the specific factors that allow it to become virulent. For example, a virulence factor could be a specific gene that the organism has that allows for toxin production, and this specific gene could be the reason for its virulence or the reason for its pathogenicity. And so if we take a look at our image down below, notice on the left hand side over here, we're showing you some diseased lungs over here that have the bacterium Mycobacterium tuberculosis, which is going to be the cause of tuberculosis or TB and leads to symptom signs and symptoms such as chronic cough and coughing up blood, chest pain, weakness and fatigue, weight loss, weight loss and loss of appetite, fever and chills, and night sweats. And so Mycobacterium tuberculosis would be considered a pathogen that has a high degree of virulence.

Now over here on the right hand side, we're showing you a healthy gut. And zooming into the healthy gut, what you'll find is a bacterium called Bifidobacterium animalis, and this bacterium is part of the healthy gut microbiome. And so it is actually a non virulent microbe, and that means that it has a very low degree of virulence, a low chance of causing infection and disease. And so notice that if this bacteria is in excess in the gut, it would only cause things like stomach gas and bloating, and rarely it would cause fever and chills. But it is not going to cause nearly as many signs and symptoms of disease as a virulent pathogen.

And so this here concludes our discussion on virulence, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

Characteristics of a pathogen that promote pathogenicity are called:

• A. Pathogen factors.
• B. Colonization factors.
• C. Infectious agents.
• D. Virulence factors.

Virulent pathogens are:

• A. Less likely to cause disease.
• B. More likely to cause disease.
• C. Only able to cause disease in weakened hosts.
• D. Always cause disease whenever inside a host.

Which of the following pathogen characteristics may be considered virulence factor(s)?

• A. Adhesins (molecules that allow a pathogen to adhere to a host cell).
• B. Capsules (protective coatings on the outside of certain bacteria).
• C. Endotoxins (damaging molecules created by certain bacteria).
• D. Proteases (damaging molecules created by certain bacteria).
• E. All of the choices are correct.

An encapsulated bacterium can be a virulent pathogen because the capsule:

• A. Resists phagocytosis.
• B. Destroys host tissues.
• C. Is composed of endotoxins.
• D. The capsule has no effect on the virulence of a bacterium.

Which of the following is a true statement?

• A. A successful pathogen never kills the host.
• B. Throughout disease evolution, the most virulent diseases have spread the fastest.
• C. A successful pathogen does not kill the host before it is transmitted.
• D. The primary goal of a pathogen is to kill the host.

Here’s what students ask on this topic:

What is the difference between signs and symptoms in infectious diseases?

Signs and symptoms are both indicators of infectious diseases, but they differ in how they are observed. Signs are objective evidence of disease that can be observed, measured, or detected by someone other than the patient. Examples include fever, rash, and swelling. Symptoms, on the other hand, are subjective experiences reported by the patient and cannot be directly observed by others. Examples include pain and nausea. Understanding the distinction is crucial for accurate diagnosis and treatment of infectious diseases.

How do communicable and noncommunicable diseases differ?

Communicable diseases, also known as contagious or infectious diseases, can spread from one person to another. Examples include the flu and COVID-19. Noncommunicable diseases, such as cancer and diabetes, do not spread between individuals. They often result from genetic, physiological, lifestyle, or environmental factors. Understanding these differences helps in developing appropriate public health strategies and treatments.

What are primary and secondary infections?

A primary infection is the initial infection caused by a primary pathogen, such as HIV. This infection can weaken the immune system, making the individual more susceptible to a secondary infection. A secondary infection is caused by opportunistic pathogens that take advantage of the weakened immune system. For example, a person with HIV may be more susceptible to infections like pneumonia. Understanding these concepts is important for managing and treating infectious diseases effectively.

What is virulence and how does it affect disease severity?

Virulence refers to the degree of pathogenicity of an organism, or how well it can cause disease. Organisms with high virulence are more likely to cause severe infections and diseases. Virulence factors are specific traits that enhance an organism's ability to cause disease, such as toxin production. For example, Mycobacterium tuberculosis, which causes tuberculosis, has high virulence, leading to severe symptoms like chronic cough and weight loss. Understanding virulence helps in developing targeted treatments and preventive measures.

What is an infectious dose and why is it important?

The infectious dose is the specific number of microbes required to establish an infection in a host. Pathogens with a high infectious dose need a large number of microbes to cause disease, while those with a low infectious dose can cause disease with only a few microbes. This concept is crucial for understanding how diseases spread and for developing effective public health strategies. For example, knowing the infectious dose of a pathogen can help in setting guidelines for exposure limits and quarantine measures.