How Microorganisms Enter a Host

How Microorganisms Enter a Host

How Microorganisms Enter a Host

  • To cause disease, most pathogens must gain access to the host, adhere to host tissues, penetrate or evade host defenses and damage the host tissues.
  • However, some microbes do not cause disease by directly damaging host tissue.
  • Instead, disease is due to the accumulation of microbial waste products.
  • Some microbes, such as those that cause dental caries and acne, can cause disease without penetrating the body.
  • Pathogens can take entry to the human body and other hosts through several routes, which are called portals of entry.

Portals of Entry

The different portals of entry for pathogens are mucous membranes, skin, and direct deposition at the lower place of the skin or membranes (the parenteral route).

Mucous Membranes:

  • Many microorganisms and viruses gain access to the body by penetrating mucous membranes lining the respiratory tract, gastrointestinal tract, genitourinary tract, and conjunctiva, a delicate membrane that covers the eyeballs and lines the eyelids.
  • Most pathogens enter through the secretion or mucous membranes of the gastrointestinal and respiratory tracts.
  • The respiratory tract is the easiest and most often portal of entry for infectious microorganisms.
  • Microorganisms are inhaled into the nose or mouth in drops of moisture and dust particles.
  • Diseases that are commonly contracted via the respiratory tract include communicable disease such as: common cold, pneumonia, tuberculosis, influenza, measles, and smallpox.
  • Microorganisms can gain access to the (GIT)gastrointestinal tract in food and water and via contaminated hands.
  • Most microorganisms that enter the body by any ways are destroyed by hydrochloric acid (HCl) and enzymes in the stomach or by bile and enzymes in the small intestine.
  • Those that survive will cause disease. 
  • Microorganisms in the gastrointestinal tract can cause hepatitis A, poliomyelitis, typhoid fever, giardiasis, amoebic dysentery, shigellosis (bacillary dysentery) , and cholera.
  • These pathogens are then eliminated with feces and can be transmitted to other hosts via contaminated water, food, fingers or hands.
  • The (GIT) genitourinary tract is a portal of entry for pathogens that are contracted sexually.
  • Some microbes that cause sexually transmitted infections (STIs) may take entry from an unbroken mucous membrane.
  • Others needs a cut or abrasion of some type. 
  • Examples of STIs are genital warts, HIV infection, herpes, syphilis, chlamydia, and gonorrhea.


  • The skin is the most largest organ of the body in terms of surface area and is an important defense against many pathogens.
  • Unbroken skin is impenetrable by most microorganisms.
  • Some microbes gain access to the body through openings within the skin, like hair follicles and sweat gland ducts.
  • Larvae of the hookworm actually bore through intact skin, and few fungi grow on the keratin in skin or infect the skin itself.
  • The mucous is a delicate membrane that lines the eyelids and covers the eyeballs.
  • Although it’s a relatively effective barrier against infection, bound diseases such as conjunctivitis, trachoma, and ophthalmia neonatorium are acquired through the conjunctiva.

The Parenteral Route:

  • Other microorganisms gain access to the body when they entered directly into the tissues beneath the skin or into mucous membranes once these barriers are penetrated or injured. This route is called the parenteral route.
  • Cuts, wounds, surgery, punctures, injections, bites, and splitting of the skin or mucous membrane due to swelling or drying can all establish parenteral routes.
  • Hepatitis viruses, HIV, and bacteria that cause tetanus and gangrene can be transmitted parenterally.

The Preferred Portal of Entry

  • Even when microorganisms have entered the body.
  • They do not cause disease or any illness.
  • The prevalence of disease depends on many factors, only one of which is the portal of entry.
  • Several microorganisms have a preferred portal of entry that is a prerequisite to their  having the ability to cause disease.
  • If they get entry to the body by another portal, disease might not occur.
  • For example, Salmonella typhi, typhoid fever, produce all the signs and symptoms of the disease when swallowed (preferred route), but if the same bacteria are rubbed on the skin, no reaction (or only a slight inflammation) happen.
  • Streptococci that are inhaled (preferred route) can cause pneumonia; those that are swallowed generally do not produce signs or symptoms.
  • Some pathogens, such as Yersinia pestis, the microorganism that causes plague and Bacillus anthracis, the causative agent of anthrax, will initiate disease from more than one portal of entry.

How Microorganisms Enter a Host


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