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Micro-organisms are everywhere. Most cause us no harm, and some are even helpful. They live in our gut and help with digestion, or recycle nutrients by decaying dead organisms.

However, there are some micro-organisms that can cause harm, and these are called pathogens.

There are four main types of pathogen.

All types of pathogen have a simple life cycle. They infect a host, reproduce themselves (or replicate, in the case of a virus), spread from their host, and infect other organisms.

They also have structural adaptations that make them successful at completing their life cycle, which enables them to cause further disease.

Diseases caused by pathogens are called communicable diseases. This means they can be transferred from one person to another.

Bacterial cells

All bacterial cells have the following features:

• no nucleus
• a cell wall made of murein, not cellulose as in plant cells
• a cell membrane and cytoplasm
• chromosome that is loose in the cytoplasm in a long loop
• may contain small loops of DNA called plasmids
• no chloroplasts or mitochondria
• exist as single cells
• sometimes have a slime capsule around the cell wall to stop them drying out
• divide by binary fission, they split into two before growing to full size and repeating the process
• may have pili to attach to surfaces, or flagella to allow them to move

Virus particle

Viruses are not considered alive because they do not complete all of the seven life processes, ie movement, respiration, sensitivity, nutrition, excretion, reproduction and growth.

We refer to strains of virus, not species. They are made up of a relatively short length of genetic material DNA which is surrounded by a protein coat.