Infectiuous diseases have ability to spread from one individual to others. It is important to understand how does diseases spread. Mode of spread of disease is also called mode of disease transmission.
This understanding helps us to curb the spread and take effective personal preventive measures.
Different pathogens spread differently.
After having infected one host, all pathogens must also transfer from one host to another or they will die when their host dies. Some pathogens always require a living host whereas others can stay in dormant stage outside the host.
Reservoirs and Carriers
Reservoirs are places where pathogens can persist over long periods of time. Reservoirs can be living organisms or nonliving sites like soil and water.
The pathogens may naturally harbor the organism because it may grow in that environment or the environment may get become contaminated with pathogens from various sources [e.g. human feces, remains of intermediate hosts].
An individual capable of transmitting a pathogen without displaying symptoms is referred to as a carrier.
A passive carrier is an uninfected individual who can mechanically transmit the pathogen. For example a health-care professional with contaminated hands from one patient can transmit a pathogen to other without himself getting infected.
An active carrier is an infected individual who can transmit the disease to others although he may not exhibit clinical features.
A pathogen may have more than one living reservoir i.e. human and animal.
Parasitic infection cycle typically consist of two hosts the definitive host [in which the parasite reaches sexual maturity] and one or more intermediate hosts in which the parasite goes through several immature life cycle stages or reproduces asexually.
Contact transmission includes direct contact or indirect contact. The agent is transmitted by physical contact between two individuals. Touching, kissing, sexual intercourse, or droplet sprays are contact transmission
Transmission from mother to child during pregnancy, birth, or breastfeeding is called vertical direct transmission.
Other kinds of direct contact transmission are called horizontal direct contact transmission.
Droplet transmission [coughin, sneezing] of a pathogen to a new host over distances of one meter or less is also a form of direct transmission.
Transmission over distances greater than one meter is called airborne transmission.
Many respiratory illnesses like flu, pneumonia cough etc are spread by droplet infections.
Indirect Contact Transmission
Indirect contact transmission involves inanimate objects called fomites. Fomites are the objects contaminated by an infected individual or reservoir.
For example, sneezing may cause droplets to land on a towel or doorknonb. Transmission occurs indirectly when a new susceptible host later touches the fomite .
The term vehicle transmission refers to the transmission of pathogens through vehicles such as water, food, and air.
Dust and fine particles are known as aerosols. These can float in the air carrying pathogens along and facilitate the airborne transmission of disease. For example, hantavirus to humans occurs like this.
[Droplet transmission over short distances is considered contact transmission but longer distance transmission of droplets through the air is becomes vehicle transmission.]
Pulmonary tuberculosis is often transmitted via airborne transmission.
Diseases can also be transmitted by a mechanical or biological vector. A vector is an animal that carries the disease from one host to another.
Mechanical transmission is facilitated by a mechanical vector, an animal that carries a pathogen from one host to another without being infected itself. For example, a fly transmitting pathogens from feces to food.
Biological transmission occurs when the pathogen reproduces within a biological vector that transmits the pathogen from one host to another. For example malaria pathogen.
Also called nosocomial infections, these infections are acquired in health-care facilities, including hospitals.
These are often related to invasive procedures that provide the pathogen with access to the portal of infection but other modes of transmission are known too.
Prolonged stay in hospitals is associated with acquiring nosocomial infections. These are surgical site infections, pneumonia, urinary tract infections, and primary bloodstream infections.