Intact skin is composed of several layers of dead, keratinized cells called the epidermis. Beneath this layer is the dermis, composed primarily of connective tissues and associated with fixed macrophages. Hair follicles, sebaceous (oil-producing) glands and sudoriferous (sweat) glands open to the outside and can sometimes act as portals into the deeper layers of the skin.
Examples of microbes that can enter and/or cause infection through intact skin include dermatophyte (skin-loving) fungi such as Trichophyton, Pitisporidium and Microsporidium, bacteria such as Propionibacterium acnes and helminth worms such as Enterobius (pinworms).
The parenteral route of infection involves break in the skin such as cuts, scrapes, puncture wounds, bites and burns. Examples of infectious agents that enter the body via this route include the rabies virus, Staphylococcus aureus, Clostridium perfringens, Pseudomonas aeruginosa and insect vector transmitted pathogens such as Plasmodium, Leishmania, and Trypanosoma.
Mucus membranes are found in the respiratory, gastrointestinal and genitourinary tracts, as well as the conjunctiva of the eye.
Respiratory pathogens such as influenza viruses, rhinoviruses and Ebola, as well as bacteria such as S. pneumoniae, H. influenzae, L. pneumophila and Y. pestis all can enter through the the mucus membranes of the respiratory tract, as can B. anthracis. All of these can be transmitted by aerosol droplets released during coughing and sneezing. Many of these can enter the respiratory tract via the nasolacrimal ducts when a person rubs their eyes.
Organisms such as C. jejuni, S. typhimurium, E. coli, H. pylori, V. cholerae and Shigella use the gastrointestinal route, carried by vehicles such as food and water. Transmission is generally via fecal-contamination.
Treponema pallidum, Neisseria gonorrhoeae, Chlamydia trachomatis, papilloma viruses, Herpes simplex II and HIV are all examples of pathogens that enter through the mucus membranes of the genitourinary tract. Though most are sexually transmitted, some can be transmitted nosocomially such as urinary tract infections caused by Proteus mirabilis, P. vulgaris and E. coli, as well as the normally nonpathogenic S. epidermidis.