Process of infection

Animals, infected humans and the environment are reservoirs for microorganisms.

Microorganisms are transmitted either directly through contact or exposure (oral) or indirectly through insects (vectors). 

Microorganisms are transmitted from one human to another through coughing and sneezing (aerosolized microorganisms) or through physical contact such as blood, touch, sexual contact or contact with contaminated materials.

Once a microorganism is in a suitable environment it attaches to the tissue (stabilizes adherence) through specific surface receptors. For example respiratory infections attach to the respiratory epithelium. This prevents the pathogen from being removed through mechanical forces such as coughing. 

Once the pathogen has stabilized itself to the tissue it can invade surrounding tissue.

This enables the pathogen to spread throughout the area of the body.

Pathogens have developed mechanisms to penetrate the body's tissue and avoid the body's non-specific and specific defence systems (inflammation and immunity).

The mechanism used typically depends on the type of pathogen (bacterial, viral or fungal).

Now the pathogen has invaded the individual's tissue it begins to multiple, provided the host has suitable conditions to multiply. 

Pathogens thrive best is a warm environment with plenty of nutrients for it to survive. 

Pathogens multiply rapidly through binary fission; asexual reproduction through separating one microorganism into two thus duplicating the DNA. 

Viral pathogens replicate in infected cells

Bacterial pathogens replicate in macrophages and other cells.

This multiplication results in localized infections which do not spread to other body parts.

However, highly invasive pathogens may invade the lymphatics, blood or internal organs.

For a pathogen to spread successfully there must be virulence factors (adhesion molecules, protection from bodys immune response and toxins available). 

Fungi are opportunistic and grow when these factors are present.

If the individual has an intact immune system it is likely the infection will stay localized.

If the individual has a damaged or compromised immune or inflammatory system the infection may spread rapidly throughout their body.