Infectious diseases: understand the essentials
Learn about microbes, transmission and prevention. Read the topic explanations, follow the chain of infection, then test what you have learned.
Bacteria: the treatment depends on the cause
Bacteria represent a diverse group of microbes. Many bacteria do not cause disease, and several types provide benefits. Others can cause infections. Some bacterial infections can be treated using antibiotics; however, having a bacterial cause does not necessarily mean that an antibiotic is required.
When studying the subject, break down three basic questions: What type of organism is involved? How is the organism transmitted? And what would make a particular infection require treatment?
Match the treatment to the cause, rather than treating all infections with the same medication.
Viruses: antibiotics are not the answer
Both influenza and the common cold are caused by viruses. Antibiotics cannot treat viral infections. Because excessive use of antibiotics can produce adverse effects and promote resistance, distinguishing between infections caused by viruses versus bacteria is critical.
Begin by determining which organism is responsible for an infection. A single symptom can occur due to a variety of diseases each with a different causative agent. Therefore, using a single symptom as a basis for selecting a medication is unreliable.
An infection represents a general term; identifying the causative agent of an infection is a separate action.
Fungi and parasites: two more groups to recognise
Fungi include organisms that can cause infections such as athlete’s foot. Antifungal medicines target fungi, whereas antibiotics used for bacterial infections act against bacteria.
Parasites utilise a host for nutrition, either internally or externally. Parasites include protozoa, worms and ectoparasites. When learning about a parasitic disease, identify the organism, the host and the mode of transmission.
The infectious agents covered here include bacteria, viruses, fungi and parasites. Each group has its own biological characteristics, which matter when considering treatment.
Transmission: trace the route from source to person
Germs can be transferred via various methods including direct contact, respiratory droplets, inhalation or sharp objects (e.g., needles). Infections may also be acquired through ingestion of contaminated food or water or through vectors (e.g., mosquitoes).
Colonisation means that microorganisms are present without causing infection. A colonised person can still transmit the organisms they carry. This is different from an infection that produces no symptoms. When following transmission, identify how the infectious agent moves from its source to another person.
Understanding the route of transmission provides guidance regarding what preventive measures should be implemented.
Prevention: look for a place to interrupt the chain
Preventive measures function at multiple stages. Washing hands can reduce the transfer of germs through contact. Improving ventilation indoors can reduce some risks associated with shared air. Vaccines can protect individuals against specific infectious agents.
Standard precautions are applied universally during patient care in healthcare settings. Additional precautions may be required depending on the pathogen(s) present and/or circumstances surrounding the infection.
Determine prevention measures based upon the pathogen, route of transmission and setting.
Resistance: microbes change and medicines lose effectiveness
Antimicrobial Resistance arises when microbial populations develop mechanisms of resistance to medicines developed to inhibit their growth. Organisms developing resistance can continue to thrive, reproduce and disperse. Resistance develops in the microbial population itself.
Proper use of antimicrobials and effective infection control practices both play key roles in preserving effectiveness of medicines: one limits inappropriate usage thereby reducing selective pressure placed on microbes; the second prevents dissemination of resistant organisms.
Preserving antimicrobial effectiveness involves proper prescribing practices and infection prevention.
Follow the chain of infection
1 · Infectious agent
The organism capable of inducing an infection. Bacteria, viruses, fungi and parasites may serve as infectious agents.
Identify the causative agent to tailor prevention efforts and treatment plans accordingly.
2 · Reservoir
The normal habitat of an infectious agent. For example, this could be a person, animal or element of the environment.
Identify the reservoir and implement appropriate strategies to minimise exposure to it.
3 · Portal of exit
The manner in which an infectious agent departs its reservoir. For example, this may involve release of respiratory secretions, blood or faeces.
Respiratory hygiene and proper handling of bodily fluids assist in limiting release of pathogens from reservoir to other hosts.
4 · Transmission
Pathogens travel from their source to another host. Transmission routes vary among pathogens: contact, inhalation, food/water or vectors (such as insects) are examples of possible modes.
Match measures to modes: handwashing can interrupt contact transmission; improving ventilation can reduce some risks associated with shared air.
5 · Portal of entry
Potential sites of entry into another host. Examples include the respiratory system, mucous membranes and compromised skin barriers.
Use of aseptic technique, appropriate protective equipment and proper wound care can help protect potential portals of entry.
6 · Susceptible host
Hosts susceptible to acquiring the infection. Susceptibility depends on immune status, pre-existing medical conditions and other factors.
Where applicable and feasible, vaccination can decrease susceptibility to specific infectious agents.
Check your understanding
A short quiz, with an explanation after each answer.
Which infections may be treated with antibiotics?
Compare two routes of transmission and explain how a preventive measure fits each route. Then use the quiz to check the distinction between the organism, the route and the host. These learning examples do not select treatment for an individual infection.