Causes and transmission of infectious diseases

Infections may be caused by bacteria, viruses, fungi, and parasites. The pathogen that causes the disease may be exogenous (acquired from an external source; environmental, animal or other people, e.g. Influenza) or endogenous (from normal flora e.g. candidiasis).

The site at which a microbe enters the body is referred to as the portal of entry. These include the respiratory tract, gastrointestinal tract, genitourinary tract, skin, and mucous membranes. The portal of entry for a specific microbe is normally dependent on how it travels from its natural habitat to the host.

There are various ways in which disease can be transmitted between individuals. These include:

• Direct contact – Touching an infected host, including sexual contact
• Indirect contact – Touching a contaminated surface
• Droplet contact – Coughing or sneezing
• Fecal–oral route – Ingesting contaminated food or water sources
• Airborne transmission – Pathogen carrying spores
• Vector transmission – An organism that does not cause disease itself but transmits infection by conveying pathogens from one host to another
• Fomite transmission – An inanimate object or substance capable of carrying infectious germs or parasites
• Environmental – Hospital-acquired infection (Nosocomial infections)

Like other pathogens, viruses use these methods of transmission to enter the body, but viruses differ in that they must also enter into the host’s actual cells. Once the virus has gained access to the host’s cells, the virus’ genetic material (RNA or DNA) must be introduced to the cell. Replication between viruses is greatly varied and depends on the type of genes involved in them. Most DNA viruses assemble in the nucleus while most RNA viruses develop solely in cytoplasm.

The mechanisms for infection, proliferation, and persistence of a virus in cells of the host are crucial for its survival. For example, some diseases such as measles employ a strategy whereby it must spread to a series of hosts. In these forms of viral infection, the illness is often treated by the body’s own immune response, and therefore the virus is required to disperse to new hosts before it is destroyed by immunological resistance or host death. In contrast, some infectious agents such as the Feline leukemia virus, are able to withstand immune responses and are capable of achieving long-term residence within an individual host, whilst also retaining the ability to spread into successive hosts.