How a disease unfolds inside a body over time, and the many different causes of disease
The natural history of disease is what happens inside a person from the moment they're exposed until the illness runs its course, assuming no treatment steps in. Almost every disease, infectious or not, follows a similar timeline:
The same timeline with CDC's stage names (the version tests draw)
No disease yet, but at risk. Ends at exposure.
→ exposurePathologic changes start with no symptoms (incubation or latency). Ends at onset of symptoms.
→ symptoms beginSymptoms present. This is the usual time of diagnosis.
The outcome.
Stage 2 has two names depending on the disease: it's called the incubation period for infectious diseases and the latency period for chronic diseases. Either way, the length varies hugely, from seconds (an allergic reaction) to decades; the incubation period for the common cold is about 2 days, while the latency period between radiation exposure and leukemia can stretch 2 to 12 years.
Infectious diseases run a second clock alongside the symptoms: when can this person spread it? The latent period is the time from infection until the host becomes infectious, and the communicable period is the whole stretch during which they can pass the agent on. The two clocks don't have to line up. Measles is communicable about 4 days before the rash appears, which is why sick-looking people aren't the only ones who spread it. (Watch the wording: a chronic disease's latency period runs from exposure to symptoms, while an infection's latent period runs from infection to infectiousness.)
A latent infection is different again: the agent stays in the body, dormant and causing no symptoms, and can wake up years later. The chickenpox virus hides in nerve cells and can come back decades later as shingles; tuberculosis can sit latent in the lungs for years before becoming active.
Not everyone who's exposed ends up with the same outcome. Line up every infected person from "no symptoms at all" to "severe, even fatal, illness" and you get the spectrum of disease. Doctors usually only see the more severe end: the "tip of the iceberg." Many milder or symptom-free cases never get diagnosed at all, yet those people can often still spread the disease to others. A person who carries and spreads an infection without showing illness is called a carrier:
Contagious during incubation, before feeling sick at all
Still contagious even after recovering from the illness
Carries and spreads the agent for months or years
Infected and contagious, but never feels sick at all
The cases doctors see and report are only the tip. Below the waterline are the mild, subclinical and undiagnosed infections, often far more of them. This iceberg phenomenon is why reported counts undercount the true burden.
In a chronic infection the agent stays present and detectable, often for life, and the person can keep spreading it (hepatitis B, HIV). In a latent infection it hides, dormant and causing no symptoms, and may wake up later (shingles, latent TB).
Caused by an organism that rarely sickens healthy people but does harm someone whose immune system is weak (Pneumocystis pneumonia in people with AIDS).
A germ getting into your body doesn't mean you'll get sick. There are three stages inside the host:
The germ is on or in the body but hasn't invaded tissue. Your defenses may destroy it.
The germ invades tissue and multiplies, in one spot (localized) or all through the body (systemic).
The infection does enough damage that you get sick.
Disease also has many different causes. Some are infectious, caused by germs or other agents (even prions, which are only proteins) that can spread from a source to a new host:
Salmonella (food poisoning), diphtheria, pertussis (whooping cough), plague, anthrax, Legionnaires' disease
Influenza, measles, herpes simplex, COVID-19 (SARS-CoV-2), rabies, hepatitis A
Histoplasmosis (a lung infection from soil with bird or bat droppings), Pneumocystis pneumonia, ringworm
Single-celled eukaryotes: malaria (spread by mosquitoes), Giardia, Cryptosporidium, toxoplasmosis
Many-celled parasites: trichinosis (Trichinella roundworms in undercooked pork or game), tapeworms
Misfolded proteins: mad cow disease (BSE) and Creutzfeldt-Jakob disease. Extremely rare, always fatal
Bacteria are single-celled prokaryotes: their cells have no nucleus. Antibiotics work on bacteria only.
Fungi, protozoa and helminths are eukaryotes. They need an antifungal or an antiparasitic drug, not an antibiotic.
Viruses can only multiply inside a living host cell (antivirals treat some). Prions are just misfolded protein, with no DNA or RNA.
Other diseases are noninfectious, and don't spread from person to person at all:
Q3Put the stages inside the host in order.
Drag to reorder (or use the arrows), then press Check.
Q4An infection that spreads all through the body, not just one spot, is called ________.