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The causes of infection and disease

It’s important to have a background understanding of what causes infection and disease. Let’s take a look at these causes now.

It’s important to have a background understanding of what causes infection and disease. Let’s take a look at these causes now.

Infections are caused by infectious agents, also known as pathogens, which invade an organism causing an immune response. Diseases are changes in an organism’s health state that impair part of or all of its normal functioning. They can be caused by a number of factors, such as genetics, toxins, radiation, or infections (Lee & Bishop, 2016).

Infectious agents

There are six main classes of infectious agents. Each varies greatly in its structure and how it causes disease. We will look at these classes in the next step.

Infectious agents can be:

  • organism specific – be able to only infect one type of organism or
  • be able to infect and cause disease in multiple different types of organisms


Some infectious agents are always present on a host, but do not cause disease. This is called colonisation, and the bacteria and fungi that reside in and on the body naturally are called our normal flora. Some bacterial species actually help to maintain the good health of an organism, while others just reside there without causing any harm unless the opportunity arises. These are called opportunistic pathogens and generally only cause infection when the host is already compromised such as through another disease, usually something that leads to suppression of the immune system (Lee & Bishop, 2016).

Causing disease

Infectious agents can cause disease in their hosts in two main ways:

  • through direct damage to host cells, or
  • through the reaction of the host’s own immune system to the pathogens.

Many infectious agents naturally create substances that are toxic to the host, or they may cause direct physical damage when attaching themselves to the host. Viruses, for example, depend on their hosts’ cells in order to replicate. They invade the cell, and replicate rapidly until they either burst out of it, inevitably killing it (lytic viruses) or they take over the host cell preventing it from functioning normally (lysogenic viruses). Also, since viruses reside inside the host cells, the only way for the immune system to fight them is to destroy the infected cells. Fighting an infection is often a delicate balance of effectively eliminating the pathogen without damaging too much of the host so that it cannot recover.

Signs and symptoms of disease

Signs and symptoms of disease vary not only in terms of how the agent damages the host, but also where. So an agent that infects the respiratory system causes respiratory symptoms eg sneezing and coughing. These symptoms can be minor and easy to manage, or major and potentially life-threatening. Whooping cough for example, which infects the respiratory system, can cause the host to cough so much and so violently that they have difficulty breathing, which can result in death. Severe diarrhoeal diseases such as Cholera can cause death via dehydration (Lee & Bishop, 2016)

Localised infections typically result in redness, swelling, pain and in some cases pus formation. Some can have specific symptoms that aid in diagnosis, such as a rash. System-wide signs and symptoms of infection may include fever, malaise (general feeling of discomfort), myalgia (muscle pain), and increased white blood cell count.

So why do we care which infectious agent is causing a disease? We will explore this in the next step.

If you are not familiar with any of the terms used here, please refer to Step 1.3 Glossary.

Your Task

Check out the glossary in Step 1.3. Are any of the terms confusing you? Let us know in the discussion below and we’ll be happy to help you.


Lee, G., & Bishop, P. (2016). Microbiology and infection control for health professionals. Melbourne, Australia: Pearson Australia.

© Griffith University
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