Stool. Organisms shed in stool include many pathogens that replicate in the lumen or epithelium of the gut, such as Shigella, G. lamblia, and rotavirus. Pathogens that replicate in the liver (hepatitis A virus) or gallbladder (S. enterica serotype Typhi) enter the intestine in bile and are shed in stool.
Blood. Pathogens spread via blood may be transmitted by invertebrate vectors, medical practices (blood transfusion, reuse of equipment), or sharing of needles by intravenous drug abusers. Bloodborne parasites, including Plasmodium spp. and arboviruses, are transmitted by biting insects.
Urine. Urine is the usual mode of exodus from the human host for only a few organisms, including S. haematobium, which grows in the veins of the bladder and releases eggs that reach the urine.
Genital tract. Sexually transmitted infections (STIs) spread from the urethra, vagina, cervix, rectum, or oral pharynx. Organisms that cause STIs depend on direct contact for person-to-person spread because these pathogens cannot survive in the environment. Transmission of STIs often is by asymptomatic individuals who do not realize that they are infected. Infection with one STI increases the risk for additional STIs, mainly because the risk factors are the same for all STIs. STIs are described in Chapters 18 and 19.
Vertical transmission. Transmission of infectious agents from mother to fetus or newborn child is a common mode of transmission for some pathogens, and may occur through several different routes. Placental-fetal transmission is most likely to occur when the mother is infected with a pathogen during pregnancy. Some of the resulting infections interfere with fetal development, and the degree and type of damage depend on the age of the fetus at the time of infection. For example, rubella infection during the first trimester can lead to heart malformations, mental retardation, cataracts, or deafness, while rubella infection during the third trimester has little effect. Congenital microcephaly and other CNS complications have been associated with Zika virus infection during pregnancy. Much is still unknown about timing of infection relative to the trimester of pregnancy. Transmission during birth is caused by contact with infectious agents during passage through the birth canal. Examples include gonococcal and chlamydial conjunctivitis. Postnatal transmission in maternal milk can transmit CMV, HIV, and HBV.
Microbes also can be transmitted from animal to human resulting in zoonotic infections, either through direct contact with or consumption of animal products or indirectly by an invertebrate vector.
Transmissionofinfectionscanoccurvia:
Insect/arthropodvectors.
Apathogencanestablishinfectionifitpossessesvirulencefactorsthatovercomenormalhostdefensesorifthehostdefensesarecompromised.
Hostdefensesagainstinfectioninclude:
Skin:toughkeratinizedbarrier,lowpH,fattyacids,normalmicrobiota
Respiratorysystem:alveolarmacrophagesandmucociliaryclearancebybronchialepithelium,IgA
Gastrointestinalsystem:acidicgastricpH,viscousmucus,pancreaticenzymesandbile,defensins,IgA,andnormalflora
Urogenitaltract:repeatedflushingandacidicenvironmentcreatedbycommensalvaginalflora
Infectious agents establish infection and damage tissues by any of three mechanisms:
They can contact or enter host cells and directly cause death of infected cells.
They can release toxins that kill cells at a distance, release enzymes that degrade tissue components, or damage blood vessels and cause ischemic necrosis.
http://ebooksmedicine.net • They can induce host immune responses that, although directed against the invader, cause additional tissue damage. Thus, the defensive responses of the host can be a mixed blessing, helping to overcome the infection but also contributing to tissue damage.