How It Works: How Pathogens Cause Disease

How It Works: How Pathogens Cause Disease

Pathogens cause harm in two main ways: by directly damaging host cells, and by releasing toxic substances called toxins. Bacteria multiply rapidly inside the body and may produce toxins that interfere with cell function — for example, the Salmonella bacterium produces toxins that irritate the gut lining, causing vomiting and diarrhoea. Viruses take a different approach: they invade host cells, hijack the cellular machinery, and force the cell to produce thousands of viral copies. When the new viruses burst out, they destroy the host cell. This cell destruction is what causes symptoms such as tissue damage, inflammation, and fever.

The key to pathogen success is reaching a target tissue in sufficient numbers. The infectious dose — the minimum number of pathogens needed to establish an infection — varies by pathogen. Norovirus can cause illness with as few as 20 viral particles, while Salmonella typically needs millions of bacterial cells. This explains why some diseases spread extremely easily while others require close, prolonged contact.

Understanding these mechanisms explains why different treatments work for different pathogens: antibiotics disrupt bacterial structures (cell walls, ribosomes) that do not exist in host cells, so they can kill bacteria selectively. Viruses lack these structures entirely, which is why antibiotics are completely ineffective against viral infections.