But what if that system overreacts? The result can be sepsis, a potentially life-threatening condition when the body's response to an infection ends up damaging its own tissues.
Around 49 million people a year contract sepsis worldwide, with roughly 48,000 deaths in the UK.
So it's exciting to reveal today that Queen Alexandra Hospital in Cosham is to host a new blood test trial that could help to predict sepsis days before a patient even shows any symptoms.
The potential of this innovative research is massive, as doctors would be able to treat the condition much earlier and prevent it escalating - saving many lives in the process, including people fighting Covid-19.
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It's taken 10 years of research by the Defence Science and Technology Laboratory (DSTL) to get to this point.
But that patient groundwork, originally begun to help service personnel survive injury and infection on the front line, means that the wider public are now set to see the benefit.
The DSTL's work will be taken forward by Presymptom Health, a government spin-out company that will further develop the test.
Staff at the QA should feel proud that the hospital will be involved in the trial, with up to 600 patients admitted with respiratory tract infections being given the option to participate.
The initial trial will last 12 months and will include samples taken from both patients and a DSTL biobank.
If it goes well, a blood test could be available for a broader NHS roll-out within two years.
How good will it be to say that Portsmouth played its part in such an important scientific advance?