APES 8.7: Pathogens and Infectious Diseases

Pathogen

A biological organism or infectious agent capable of causing disease in a host. Examples include viruses, bacteria, fungi, protozoa, prions, and viroids.

How do pathogens differ from all causes of disease?

Pathogens cause disease through infection. Other diseases can result from noninfectious causes such as genetic mutations, autoimmune reactions, or exposure to pollutants.

Pathogenicity

Pathogenicity is a pathogen's capacity to cause disease. It depends on both infectivity, the ability to infect a host, and virulence, the severity of disease caused.

What are the major routes by which pathogens are transmitted?

Transmission may occur through direct contact or respiratory droplets, indirect contact with contaminated objects or surfaces, or living vectors such as mosquitoes and ticks.

Basic reproduction number, $R_0$

$R_0$ is the expected number of secondary cases produced by one infected individual in a completely susceptible population. Values greater than 1 indicate that an infection can potentially spread.

What are bacteria, and why are most not pathogenic?

Bacteria are single-celled prokaryotes. Most are harmless or beneficial, including gut microbes that contribute to digestion; only a small fraction cause infectious disease.

How do bacterial pathogens differ from viral pathogens in cellular organization?

Bacteria are cellular prokaryotes with their own metabolic machinery. Viruses are acellular infectious particles that must use host-cell machinery to replicate.

How do viruses replicate inside host cells?

A virus delivers its genome into a host cell and redirects the cell's machinery to make viral components and new virions. The resulting particles can leave the cell and infect additional cells.

Protozoan pathogen

A protozoan pathogen is a single-celled eukaryote that causes disease, often as a parasite. Examples include the organisms responsible for malaria, giardiasis, toxoplasmosis, and sleeping sickness.

What are waterborne pathogens?

Waterborne pathogens are disease-causing organisms transmitted through contaminated water. They commonly include bacteria, viruses, and protozoa from human or animal waste.

How can polluted water spread infectious diseases?

Pathogens released in sewage or other biological waste can contaminate surface water or groundwater. People may become infected by drinking the water, using it for food preparation, or contacting and accidentally ingesting it.

Why does untreated sewage increase the spread of infectious disease?

Untreated sewage can contain bacteria, viruses, and protozoa. Discharging it into waterways allows pathogens to persist and reach drinking-water supplies, recreational waters, crops, and new hosts.

What is the purpose of wastewater treatment?

Wastewater treatment removes or reduces solids, organic matter, nutrients, and pathogens before treated water is discharged or reused. This protects human health and aquatic ecosystems.

What occurs during primary wastewater treatment?

Primary treatment is mainly physical. Screening removes large debris, and settling allows suspended solids to form sludge that can be removed from the wastewater.

What occurs during secondary wastewater treatment?

Secondary treatment uses microorganisms to biologically decompose dissolved and suspended organic matter. Aeration commonly supplies oxygen so aerobic microbes can break down waste, reducing biochemical oxygen demand.

What occurs during tertiary wastewater treatment?

Tertiary treatment provides advanced removal of remaining contaminants, such as nutrients, fine particles, and pathogens. Processes may include filtration, chemical treatment, or disinfection.

How do primary, secondary, and tertiary wastewater treatment differ?

Primary treatment removes large and settleable solids; secondary treatment uses microbes to decompose organic matter; tertiary treatment further removes nutrients, fine pollutants, and pathogens through advanced processes.

Why is disinfection important in wastewater treatment?

Disinfection, using methods such as chlorination, ultraviolet light, or ozone, inactivates remaining pathogens. It reduces the risk that treated effluent will transmit infectious disease.