Symbiotic Relationships and Disease

Duration of Sickness

Diseases are characterized by duration and intensity. Acute illness is rapid in onset, lasts for a short period of time, and responds well to treatment. Chronic diseases develop slowly and are prolonged over years, and many have no cure. Latent disease is not clinically active, but the pathogen remains dormant inside the host body.

The length and intensity of a disease are important considerations. Diseases that progress rapidly, are severe, and last for a short period are considered acute. Patients with acute illness tend to respond well to treatment and typically return to their normal preinfection health with no long-lasting complications. An example of an acute illness is preseptal cellulitis, a skin infection around the eyes commonly caused by Staphylococcus or Streptococcus species. Treatment with a penicillin- or sulfa-based antibiotic for one week resolves all symptoms.

In contrast, diseases categorized as chronic last a prolonged period of time. The intensity of chronic diseases develops slowly and tends to progressively worsen. Most have no cure, and patients will live with the chronic disease for life. Hepatitis C infection is an example of a chronic disease. The hepatitis C virus is transmitted through contaminated blood (such as from unsterilized needles used for tattooing or drugs) or through blood transfusion. It causes liver inflammation, which over time leads to scarring and prevents the liver from functioning normally. There is no cure or vaccine for hepatitis C. Like in many patients with chronic disease, people with hepatitis C infection may experience periods of remission, where their symptoms weaken or disappear for stretches of time before they return.

A latent disease is a disease that is not clinically active, or without signs or symptoms of disease in the host; the pathogenic agent remains dormant within a cell of the host body. A well-known example of latent disease involves the varicella-zoster herpes virus, which is the causative agent of chickenpox. Once the chicken pox infection resolves, the zoster virus retreats to the dorsal root ganglia (the cell bodies of the sensory nerves in the spinal cord), where it remains dormant (latent). Later in life, possibly because of changes in the immune system of the host, the virus can become reactivated in the form of shingles along a specific dermatome (the area of skin supplied by a single nerve root).

Subacute diseases are intermediary between acute and chronic in terms of duration and intensity. An example of a disease that has classically been diagnosed in acute and subacute forms is bacterial endocarditis, or heart tissue infection. Acute endocarditis indicates a destructive infection of a previously healthy heart valve by a highly virulent organism, such as Staphylococcus aureus. These infections can lead to death within days to weeks even with treatment and surgery. Less virulent organisms, such as Streptococcus viridans, cause a subacute infection in previously diseased heart valves. Such cases may appear suddenly but persist for weeks to months. With treatment, most patients with subacute endocarditis recover.

The intensity and duration distinctions among acute, subacute, and chronic diseases are flexible and exist on a continuum with no definitive boundaries between them. There is no specified number of days of infection that universally distinguishes acute disease from subacute. These are qualitative and not quantitative distinctions. Another related term is subclinical, which refers to infections that resemble acute or subacute disease but produce no clinical symptoms in the patient. In a subclinical infection, the pathogen enters the host, replicates, and elicits an immune response that clears the infection without apparent symptoms or harm to the host.

Categorization of Diseases

Pathogen Infectious Communicable Transmission Intensity and Duration
None; lung cancer No No Not applicable Chronic
HIV Yes Yes Direct Chronic
Influenza Yes Yes Direct/indirect Acute
Salmonella typhi Yes Yes Indirect, oral-fecal Acute
Streptococcus pneumonia Yes No Not applicable Acute
Mycobacterium tuberculosis Yes Yes Direct Acute to chronic

Pathogens can be categorized into infectious and noninfectious groups. Infectious pathogens are further categorized by whether they are communicable or not. Other important considerations are transmission, intensity, and duration of the disease.