Process of Infection

Four stages of progression (from point
of view of microorganism):
1.Colonization
2.Invasion
3.Multiplication4.Spread

Infectious microorganisms exist in reservoirse.g.
environment (contaminated water, soil), animals or
infected human

Transmitted via direct contact, or indirectly by
vectors(insects) –bites/stings/passive transfer;
direct exposure –faecaloral transmission through
food/water (salmonella, cholera) or soil (tetanus)

Infectious microorganisms exist in reservoirse.g.
environment (contaminated water, soil), animals or
infected human

Transmitted via direct contact, or indirectly by
vectors(insects) –bites/stings/passive transfer;
direct exposure –faecaloral transmission through
food/water (salmonella, cholera) or soil (tetanus)

Human-to-human transmission by aerosolized

microorganisms in droplets (coughing/sneezing) –

primary means of respiratory tract infections;

physical contact (sexual contact, blood

transfusion, direct contact contaminated materials)

▪ After deposit in receptive environment  -

microorganism stabilizes adherence to the tissue

through specific surface receptors e.g. infectious

agents causing respiratory tract infections bind to

molecules on respiratory epithelium – this helps

protect removal of the pathogen by mechanical

forces e.g. coughing mucus

INVASION

Infectious agent can invade surrounding tissues/other
sites

Developed mechanisms to penetrate tissues & avoid
host’s nonspecific and specific defences(inflammation
& immunity)

MULTIPLICATION

Warm, nutrient filled host environment

 Undergo rapid multiplication – replicate within infected

cells (viral pathogens); replicate in macrophages &

other cells (bacterial pathogens)

SPREAD

SPREAD

Produce localized infections without spread to other  regions of the body

Others highly invasive  - may enter lymphatics, blood, internal organs

Successful spread - relies on virulence factors –

adhesion molecules, toxins & protection against host’s inflammatory and immune system – Fungi are opportunistic

If host has:

Intact immune system – microorganism remains localized

Immune or inflammatory system compromised – infection may rapidly spread