Infection

A diversity of living organisms has developed the ability to live within our bodies. The broad classes of these include:

• Viruses, small particles that enclose a viral genome that must reproduce within cells.
• Bacteria, simple cells that can often divide rapidly.
• Fungi, more complex cells or branching strands of cells.
• Parasites, a broad term for organisms that invade our body that are not any of the above classes. They can include single-cell ‘protists’, and parasitic animals such as parasitic worms, and parasitic mites.

Organisms that are only visible using a microscope, such as viruses, bacteria and many fungi and single cell parasites, are often called ‘microbes’.

Infections can occur in many different parts of our body. Some organisms, such as the scabies mite, invade only a few millimetres into the outer layers of our skin. Viruses can only grow and reproduce within cells.

Some bacteria, fungi and parasites also grow within cells. Others can live within certain tissues in our body, or in cavities such as our lungs or mouth. Some diseases can occur without direct contact with the microbe, such as illness caused by toxins released by bacteria into foods.

There are many different ways that infectious agents make us sick. These include:

• Damaging cells or tissues within our body directly, such as a virus killing its host cell.
• Secreting toxins that damage cells.
• Forcing cells to behave abnormally, such as dividing uncontrollably.
• Competing for nutrients or other resources with our own cells.
• Physically obstructing the normal function of an organ.
• Triggering harmful immune responses or inflammation.

The immune response is critical to prevent infections spreading. People with immunodeficiencies may not mount an appropriate immune response and can suffer overwhelming illness from microbes that normally cause mild or no infection.

The symptoms of an infection can be caused by:

• The infectious agent itself, or toxins it releases.
• The immune response to that infection, especially inflammatory responses

The duration of an infection can vary. An infection can be:

• Acute, meaning the infectious agent is soon removed, although the damage it causes may take longer to resolve.
• Chronic, meaning the infectious organism persists within our body long-term, avoiding immune clearance.

Some infections are chronic, but periodically reappear in an acute form.

Our immune system protects us from many infections. However, some infectious organisms have developed strategies to evade our immune defenses. These can include:

• Changing proteins on their surface (or the surface of an infected cell) to evade specific immune responses.
• Hiding within cells.
• Producing a dormant form that can re-awaken when host immunity is weakened.
• Residing in parts of the body that are poorly accessed by the immune system, such as the skin surface or brain.

Our researchers are discovering how particular infectious organisms evade immune detection. This is revealing new strategies for enhancing the immune response to clear the infection, and also new approaches to designing vaccines to prevent infection.

Two important ways to prevent contacting an infectious disease are:

• Avoiding exposure to the infectious agent, such as avoiding mosquito bites to prevent malaria, and washing hands to reduce the spread of influenza.
• Inducing a protective immune response, such as through vaccination, that prevents the infection establishing.

The smallpox virus was the first infection to be completely eliminated from humans worldwide, aided by a global vaccination program. Our researchers aspire to contribute to the elimination of other infections, particularly malaria.

The ideal treatments for infections kill the infectious agent without harming the body’s own cells. Many treatments target molecular differences between microbes and humans.

The broad classes of medicines used to treat infections are:

• Antivirals
• Antibiotics, to treat bacterial infections
• Antifungals
• Antiparasitics

Our researchers are investigating many new approaches to treating disease. An important focus is to identify molecules unique to an infectious agent that can be targeted by new medicines developed using medicinal chemistry.

Our researchers are also developing new approaches to treating infections, particularly chronic infections, by boosting the immune response to the infectious agent.

For some infectious diseases, treatments also involve reversing or resolving the damage caused by the infection. For example, people with gastrointestinal infections who are dehydrated are given additional fluids.

As infectious organisms proliferate, some acquire genetic changes. Occasionally these allow an organism to become resistant to a particular treatment. When that treatment is given, the resistant microbes can continue to grow in our bodies, replacing the treatment-sensitive organisms. Resistance is more likely to develop when people do not take a full course of treatment, such as an antibiotic course.

Over time a drug-resistant strain of a disease can become the predominant strain in the community. When more than one type of treatment for an infection is available, a strain of microbes resistant to one treatment may be sensitive to another.

Over time, multi-drug resistance can develop, rendering many treatments ineffective. Increasing rates of multi-drug resistance are limiting the options for treating many types of infection, including malaria and tuberculosis.

Our researchers aim to develop new treatments for infections using approaches that minimise the likelihood of resistance developing. By investigating the basic biology of infections, they are revealing how drug resistance occurs, and designing treatment strategies that are more difficult to evade.

Treatments that boost the immune system’s clearance of infections are another approach that may overcome problems posed by multidrug resistant infections.