How microorganisms cause disease and how infections spread

A pathogen is any agent that can cause disease. Pathogens can be cellular (bacteria, fungi, protozoa, worms) or acellular (viruses). The big dividing line among living cells is the nucleus: prokaryotes such as bacteria have none, while eukaryotes such as fungi, protozoa and humans do.

Viruses deserve a second look. Each one carries either DNA or RNA, never both, inside a protein coat. A virus cannot do anything on its own. It must get inside a host cell and use that cell's machinery to copy itself, which is why viruses are called obligate intracellular parasites.

Pathogenicity is a microbe's ability to cause disease, and virulence is how severe that disease tends to be. Most infections follow four stages:

1. Exposure: the pathogen meets the host, usually at a portal of entry such as the respiratory, digestive or genitourinary mucous membranes, or a break in the skin.
2. Adhesion: it attaches to host cells, often using surface proteins called adhesins.
3. Invasion: it spreads into tissues or enters cells.
4. Infection: it multiplies, either in one spot or throughout the body.

Damage comes in different ways. Some bacteria release toxins, such as the Shiga toxin made by certain E. coli strains. Viruses take over the cells they enter, turning them into virus factories that release new viruses to infect more cells.

No. Your skin, gut and other surfaces carry huge communities of microbes called the normal microbiota (older books say normal flora). Many of them are considered harmless, and some help by keeping other microbes in check.

A primary pathogen can cause disease in a healthy person. An opportunistic pathogen causes disease only when it gets a chance. That happens when host defenses are weak, or when it reaches a part of the body where it does not belong. For example, E. coli that lives harmlessly in the large intestine can cause a urinary tract infection if it reaches the bladder.

The yeast Candida is another example. It is normally held back by neighboring bacteria. If antibacterial drugs kill those bacteria, Candida (a fungus, so unaffected by them) can overgrow.

Antibiotics fight bacterial infections by killing bacteria or stopping their growth. They do not work on viral infections such as colds or the flu. Viruses have no cell of their own to attack: they borrow the host cell's machinery.

Antiviral drugs work differently: they block a specific step in a virus's life cycle. That step might be entry into the cell, copying the viral genome, or release of new viruses from an infected cell. Because viruses rely on host machinery, safe targets are harder to find. Fungi, protozoa and worms are eukaryotes like us, so they need their own drug types too.

Taking antibiotics when they are not needed can contribute to antibiotic resistance, meaning the drug no longer affects the bacteria. Antibiotics can also cause side effects, including yeast infections.

Epidemiologists describe spread as a chain of infection. The pathogen leaves its reservoir through a portal of exit, travels by a mode of transmission, and enters a susceptible host through a portal of entry.

• Reservoir: where the pathogen normally lives and multiplies, such as people, animals or soil and water. A carrier is infected but has no symptoms, yet can still spread it.
• Mode of transmission: direct or indirect. Direct spread is touching or large droplets sprayed a few feet by coughing and sneezing. Indirect spread uses airborne particles, contaminated objects called fomites, food or water, or vectors such as mosquitoes and ticks.
• Susceptible host: how likely a person is to get sick depends on genetics, immunity and general defenses.

Breaking any link stops the chain: treating infected people, isolating them, cleaning contaminated objects, controlling vectors with bed nets, or vaccinating to make hosts less susceptible.