Direct Person-to Person Transmission

Sexual Transmission

Intimate sexual contact provides opportunities for transmission of several types of infectious agents:

  1. The infectious agent doesn't have to survive in the external environment.
  2. It doesn't need a large population of potential hosts to sustain transmission and survival.
  3. It doesn't need an animal reservoir to sustain its survival.




Trichomonas vaginalis




Bacterial vaginosis

Chancroid (Haemophilus ducreyi)

Lymphogranuloma venereum


Human papilloma virus (HPV)

Genital herpes

Viral hepatitis (B> C > [A])




Estimates of New HIV Infections, by Race/Ethnicity, Risk Group,

and Gender for the Most Affected US Populations, 2009


Source: Prejean J, et al. Estimated HIV incidence in the United States, 2006-2009. PLoS One 2001;6(8):1-13.


Per Act Risk of HIV Transmission for Various Behaviors

These estimates are rough approximations gleaned from a number of studies. There are several factors that can modify risk, such as viral load in the infected partner. For an excellent discussion of HIV risk see Safer Sex Methods at the HIV-Insite from UCSF.


Transmission Probability (Attack Rate)

Blood Transfusion


Penile vaginal intercourse


Penile-anal intercourse

Oral sex 

Injecting drugs

Health care (sharps injury)

Mucous membrane exposure to infected blood





0.0005-0.0009 male-to-female

0.0003-0.0001 female-to-male


Much lower, but definitely possible




Rare - can occur via blood contact

Virtually non-existent


   Toggle open/close quiz question

Which of the following are established mechanisms for transmission of HIV?


[mark all correct answers]




Disease can be transmitted through kissing. A wide variety of viruses that cause upper respiratory tract infections (influenza, sore throat, cold, laryngitis, tracheitis, etc.) can be transmitted through kissing. In addition, the human papilloma virus can be transmitted through kissing, particularly oral sex, and this can contribute to the risk of oral cancers. Other diseases that can be transmitted through kissing include mononucleosis (a virus) and Streptococcus (a bacterium).

Perinatal Mother-to-Child Transmission (PMTCT; Vertical Transmission)

There are several diseases that can be transmitted from mother to child in the perinatal period. Cytomegalovirus is the most common, but HIV is also transmitted vertically with an incidence of around 25% with preventive measures.

These diseases include:




Chagas Disease (Trypanosoma cruzi)

Rubella virus




Hepatitis B

Coxsackie virus

Epstein-Barr virus,

Varicella-zoster virus (chicken pox)

Human parvovirus)

West Nile virus

Eastern equine encephalitis

Yellow fever


Mother to child transmission of HIV can occur before, during, or after delivery (15-40%). Transmission is rare during early pregnancy, but relatively frequent in late pregnancy & during delivery.

Breastfeeding increases risk by approximately 15%. The wide range in risk estimates are due to the influence of other factors that increase the risk of vertical transmission:

From: Paintsil E and Andiman WA: Update on successes and challenges regarding mother-to-child

transmission of HIV. Curr Opin Pediatr. 2009 February ; 21(1): 94–101.


Basic principles of mother-to-child transmission of HIV

"Without any intervention to prevent transmission, the rate of MTCT of HIV is estimated at 12-40% (3). MTCT of HIV can occur before, during, and after birth. The relative contribution of each of these modes of perinatal transmission is not well defined (4). The risk factors associated with MTCT are illustrated in Table 1. In resource-limited countries, breast feeding contributes significantly to MTCT."


Prevention of mother-to-child transmission

"Current interventions to prevent MTCT target the late intrauterine and intrapartum periods, when most transmission events occurs. Administration of antiretroviral drugs to an HIV infected mother and her infant, careful management of labor and delivery (with elective caesarean delivery for women with high HIV viral loads) and avoidance of breast feeding have reduced the rate of MTCT to less than 2%."



"While the birth of an HIV-infected child in a resource-rich country is now a sentinel health event, in most resource-limited countries the birth of an HIV-infected child continues to be the status quo. Comprehensive PMTCT including ARV treatment for HIV-infected women and HIV-infected children should be paramount in resource-limited countries."

Needle Injection & Occupational Sharps Exposure

There are many organisms that can be transmitted to health care workers in a clinical setting. In this section focuses primarily on occupational "sharps" injuries (direct contact) to infectious agents, including HIV, hepatitis B, and hepatitis C, although other pathogens might also be transmitted by this route. We could also include contaminated blood products and shared needles among intravenous drug users, since these also involve exposure via breaching of the skin barrier barrier. However, one might also consider these to be common vehicle modes of transmission.

Occupational injuries that result in exposure to HIV, hepatitis B & C and other agents are common. It is estimated that there are 380,000 needle stick injuries per year in hospitals, and  many more in non-hospital settings. As of 2001, CDC had reports of 57 documented cases & 138 probable cases of seroconversion after occupational exposures in health care. While these injuries warrant great concern, the absolute risk of clinical infection is low. For example, it is estimated that the risk of seroconversion after exposure to HIV-infected blood from a needle stick injury in about 0.003 or 0.3%. The risk of seroconversion after mucous membrane exposure to infected blood is estimate at 0,09%, and the risk of conversion after exposure of non-intact skin to contaminated body fluid is too low to estimate. Nevertheless, these exposures are all preventable and can have dire consequences, and appropriate precautions should always be taken. For more information see the CDC information regarding healthcare-associated injuries.

The primary precautions include:

When occupational exposures occur, post-exposure prophylactic treatment should be strongly considered.

The best practice is to exercise "universal precautions" for all patients.

Universal Precautions

  1. Barrier protection should be used at all times to prevent skin & mucous membrane contamination with blood, body fluids.
  2. Wear gloves if there is potential for contact with infectious material.
  3. Wear a face shield & [protective clothing during procedures that could generate droplets of blood or body fluids.
  4. Wash hands & other skin surfaces immediately after contact with blood, other body fluids, or contaminated items.
  5. Wash hands after removing gloves.
  6. Take precautions to avoid "sharps" injuries: needles, scalpels, etc..Used "sharps" should be disposed of in a puncture-resistant container marked with biohazard symbol.

Prevention of HIV

Cautious partner selection and practice of safer sex are important steps towards prevention of HIV transmission. In addition, the following comments and measures are relevant.

US National HIV/AIDS Strategy

The major recommendations are outlined here. The full report can be obtained from

Reducing New HIV Infections

Increasing Access to Care and Improving Health Outcomes for People Living with HIV

Reducing HIV-Related Disparities and Health Inequities

Achieving a More Coordinated National Response to the HIV Epidemic

Skin to Skin Transmission

Some disease agents can be spread by direct skin to skin contact, such Tinea capitis, the fungus that causes ringworm, Tinea pedis, the fungus that causes athlete's foot, and impetigo. However, these disease are probably more often spread via fomites.

Human Bites

Bites from humans also represent a direct method of disease transmission from human to human. Human bites frequently become infected (10-15%), because of large numbers of bacteria in saliva. Diseases acquired as a result of animal bites would be considered to represent zoonotic transmission.


How is Ebola transmitted?


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