What Are Diseases and Why Do We Need Defences?
Every day, your body is under attack. Billions of microscopic organisms — bacteria, viruses, fungi, and parasites — exist in the environment, and many of them can cause harm if they get inside your body. These disease-causing agents are called pathogens.
A microorganism (such as a bacterium, virus, fungus, or parasite) that causes disease in a host organism.
A condition in which the normal functioning of the body is disrupted, often caused by infection, genetic factors, or environmental influences.
Without a defence system, even a minor infection could be fatal. Fortunately, the human body has evolved a remarkable, multi-layered immune system that works around the clock to detect, neutralise, and remember threats. Understanding how this system works helps us appreciate vaccines, medicines, and healthy lifestyle choices.
Think of your body's defence system like a medieval castle. The walls and gates are your first line of defence. Guards patrolling inside are your second line. And a specialised intelligence unit that remembers past enemies is your third line — the adaptive immune system.
Types of Pathogens
Not all pathogens are the same. They differ in structure, size, and the way they cause disease.
| Pathogen Type | Example | How It Causes Disease |
|---|---|---|
| Bacteria | Salmonella, Streptococcus | Release toxins; damage cells directly |
| Virus | Influenza, HIV, SARS-CoV-2 | Invade host cells and hijack their machinery to replicate |
| Fungus | Candida (thrush) | Overgrow tissues; release irritating chemicals |
| Parasite | Plasmodium (malaria), tapeworms | Consume nutrients; damage organs |
A single-celled prokaryotic microorganism. Some bacteria are beneficial, but pathogenic bacteria cause disease by releasing toxins or directly damaging tissues.
A non-living infectious agent consisting of genetic material (DNA or RNA) surrounded by a protein coat (capsid). Viruses can only replicate inside a living host cell.
Viruses are not cells and are not alive in the traditional sense — they cannot carry out metabolic processes on their own. This is why antibiotics (which target bacterial cell processes) have no effect on viruses.
A very common error is thinking that antibiotics cure viral infections like the flu or COVID-19. Antibiotics only work against bacteria, not viruses. Taking antibiotics unnecessarily contributes to antibiotic resistance — a serious global health problem.
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