Infectious Diseases - Basic Principles

What are Notifiable Diseases?

Infectious diseases are diseases that occurs when pathogens (disease-causing agents) get into our bodies. They are usually in the form of bacteria and viruses but may also include parasites. Consider common diseases that require immunization such as measles, mumps, tetanus and chicken pox. These are all due to infectious pathogens. Some infectious diseases are required to be reported to the Centers for Disease Control (CDC) so that their incidence, prevalence and distribution can be monitored. These are known as notifiable (reportable) diseases. The CDC lists several notifiable diseases on their website including measles, polio, hepatitis, salmonellosis, and HIV. If you are diagnosed with any of these, your doctor will report it to the CDC. This will help the CDC to maintain appropriate statistics to track, monitor and control the spread of the disease. The occurrence of an infectious diseases may be described as an outbreak, endemic, epidemic, or pandemic depending on its distribution.

Difference Between Outbreak, Endemic and Pandemic

It is an outbreak if the disease occurs in a limited area and then goes away after a short time. For example, people who ate contaminated food at a wedding may get sick. However, the disease does not spread or last for a long time. The problem goes away quickly once those limited number of individuals are healed. An endemic disease is a disease that is naturally present in the population. That is, it is not caused by an outbreak from somewhere else, it’s just naturally present. For example, chickenpox is always present in the United States. We just have to make up our minds and accept that it is here living with us and that there is little we can do to totally eradicate it. Instead, the best thing we can do is to make sure we are vaccinated against it. An epidemic occurs when there is a high prevalence of a disease in a wider region than an outbreak. For example, an unusually large number of people in Mercer county, or in Ohio coming down with influenza. A pandemic is when the disease has spread across different countries.

Sources of Infectious Diseases

Common sources of disease transfer include humans, animals, insects, soil and water. These are referred to as reservoirs for the infectious agents. Some humans may be reservoirs without showing any symptoms (asymptomatic). Hence, lack of symptoms does not guarantee that you cannot get a disease from that person.

How Are Disease Agents Transferred?

Infectious diseases can be transferred either vertically or horizontally. Vertical transmission is transmission of disease from mother to child. Horizontal transmission is transmission by direct or indirect contact. Example of direct contact would be touching, kissing, inhaling cough, or having sexual intercourse with someone, or getting infection from a bite from an insect or animal. By the way, diseases transmitted by human contact is referred to as contagious or communicable diseases. Indirect transmission of disease occurs when you touch an inanimate object (fomite) and pick up the pathogen (e.g. bacteria or virus) as a result. This would happen if you use a dirty dishcloth to wipe your plate before eating, or if you touched a surface that someone sneezed on and then you put your hand in your mouth. Transfer of pathogens may be via one of several portals i.e.

Respiratory tract e.g. picking up germs from someone’s sneeze or cough
Gastrointestinal tract e.g. getting food poisoning via fecal-oral route
Genitourinary tract e.g. getting a sexually transmitted disease
Skin e.g. getting a cut or using an infected needle

Interestingly, many diseases are caused by infection in healthcare environments. These are called nosocomial infections or healthcare-associated infections (HAIs). HAIs originate in the hospital but may not show themselves until after the patients are discharged and receiving extended care at home. According to the CDC (2019) one in 31 patients in US hospitals get infected in hospitals daily, leading to somewhere in the region of 100,000 deaths per year. HAIs usually occur as a result of:

Central line associated blood stream infection: Catheter placed in neck, chest or groin to administer medicine or collect blood
Catheter-associated urinary infection: Catheter inserted in the bladder to collect urine
Ventilator-associated pneumonia: Tube placed in the mouth, nose or neck to give oxygen
Surgical site infection: Infection that occurs at the site of infection

In many cases, the patient is discharged with the catheter still in, which increases the chance of infection during home-care. Pathogens causing disease in HAI cases may come from normal microflora on the patient. Normal microflora do not generally cause harm but may become pathogenic due to illness and immune-deficiency of the patient. Hence they are called opportunistic pathogens. One example of an opportunistic bacteria that is part of your normal microflora is the antibacterial-resistant organism called Methicillin-resistant Staphylococcus aureus (MRSA). Like other flora, MRSA is naturally present on the skin and in the nose of carriers but do not do any harm until there is a drop in immune resistance. The infection is associated with skin abscess and inflammatory symptoms.

Other sources of pathogens apart from normal microflora include the hospital environment, dirty equipment, contact with, and activity of healthcare providers.

Potential Bacteria Mode of Action in Disease

Kill cells
Disrupt cellular functions
Crowd tissues and disrupt normal function
Produce toxins
Cause immune system to over-react

Potential Virus Mode of Action in Disease

Cell lysis
Cellular disruption
Trigger abnormal cell growth

How Infections Are Avoided

Health care providers help to prevent HAIs by following best practices such as wearing personal protective equipment (PPE), hand washing, disinfecting the patient environment, sanitizing equipment, and handling garbage and textiles in such a way that disease is not spread. These same strategies can be applied to the prevention of just about any infectious disease in or outside of the hospital environment. Other general infection-prevention strategies include isolating infected individual from the rest of the population, quarantining healthy individuals suspected of having the infection, and vaccination.