Infection

Colonization

_{Infectious microorganisms exist in reservoirs e.g.environment (contaminated water, soil), animals or infected human}

_{▪ Transmitted via direct contact, or indirectly by vectors (insects) – bites/stings/passive transfer;} 

_{direct exposure – faecal oral 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

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