Infection

Pathogens enter and multiply, destroying cells and causing illness that may develop into disease.

The infection spreads in a cyclic chain.

Pathogens like bacteria, viruses, and parasites—start the chain.

These pathogens live, grow, and multiply in reservoirs, such as humans, animals, and the environment.

The reservoir determines the pathogen's portal of exit. Saliva, feces, and blood are human exits.

The mode of transmission can be direct or indirect.

Physical contacts like kissing, and sexual contact, and droplets like sneezing, coughing, and talking carry the virus directly.

However, mosquitoes or inanimate objects like door handles can spread the disease indirectly.

The pathogen enters a susceptible host through the portal of entry through the respiratory tract or fecal-oral route and multiplies in cells and tissues.

Finally, the host's susceptibility may depend on genetics, specific immunity, and other nonspecific factors that affect infection resistance and pathogenicity.

Notably, the cycle continues unless there is a break in the chain, for example, through infection control and contact tracing.