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How do Viruses and Bacteria Cause Disease?


Published: 29 September 2016

Cover image for article: How do Viruses and Bacteria Cause Disease?

A video explanation of how viruses and bacteria cause disease.

Which Germ is it?



Viruses, however, are 10 to 100 times smaller than bacteria. They cannot be seen in stains except with an electron microscope and they can only grow in artificial media containing living cells. They have a different structure (morphology) to bacteria and are not susceptible to antibiotics.


Practical Challenge 1

Try listing all the viruses you know, the diseases they cause, and their associated symptoms to remind you of their variety and complexity.

Invading the Host

Viruses tend to target specific tissues (cells) in the host.

For example, the influenza virus has a predilection for the respiratory tract, hepatitis viruses target the liver, poliovirus targets the motor neurones of the spinal cord and rotavirus multiplies in the gut. Symptoms of a viral infection may be subtle and nonspecific or specific and suggestive of the causative agent.

Dengue virus, Ross river virus, measles and rubella infections are associated with fever and a widespread red rash, chicken pox and herpes simplex viruses are associated with blistering, often localised, rashes; and hepatitis viruses cause liver damage and jaundice.


These bacterial infections are often manifested by the presence of pus wherever the bacteria settle, and systemic symptoms such as fevers, chills, pain, swelling and loss of function occur when bacteria invade and multiply.

Bacteria and Viruses Don’t Have a Brain

For bacteria and viruses to cause disease they must first colonise the patient.

Once they have invaded the host they can multiply readily. If the host is immune to the virus or bacteria due to previous exposure or vaccination, these organisms may not be able to cause disease. The person’s immune system detects their presence and eliminates them.

However, if there is an imbalance in the host (e.g. the person is immune-compromised or non-immune), the microbe is virulent. Small numbers are capable of causing illness, however if there is exposure to large numbers of microbes that can paralyse or overcome the immune system, and the host is a nutritious target, then disease may result.

Bacteria and viruses do not have brains to organise their attack on the host. Their threat lies in their ability to multiply quickly (in around 20 minutes for most bacteria). Moreover, they can rapidly change their molecular structure by mutation or selection to ensure their survival in adverse conditions.

Protecting Yourself and Your Patients


We can protect ourselves from many viral and bacterial diseases by using the vaccines that have been developed to boost our immunity so that we are not vulnerable when exposed to these microbes. We can also practice avoidance techniques by using personal protective equipment (PPE) to protect our skin and mucous membranes from contact with these organisms. Thirdly, we should wash our hands regularly to ensure that any microbes that we have picked up inadvertently from the environment do not get a chance to be inhaled, inoculated or ingested.

I challenge you to incorporate these strategic actions into your practice and help others to understand and adopt similar safe living strategies.

Practical Challenge 2

Compare the viral infections you know with the common bacterial infections you see in practice (e.g. abscesses, wound infections and urinary tract infections). Think about the similarities and differences between bacterial and viral infections.


Portrait of Joan Faoagali
Joan Faoagali

Joan Faoagali qualified as a specialist pathologist in Dunedin, NZ, and held roles including Director of Microbiology at Christchurch Hospital, Royal Brisbane Hospital, and the Princess Alexandra Hospital. She held a high interest in the prudent use of antibiotics, prevention of cross-infection, vaccination and the appropriate use and interpretation of pathology tests. She sadly died in January, 2017. See Educator Profile

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Genny Herbert
24 Jun 2019

Evidence based research.