Kissing involves the intimate exchange of saliva, close respiratory proximity, and mucosal contact, creating pathways for pathogen transmission. The human mouth harbors a complex community of bacteria and viruses, and deep kissing can transfer as many as 80 million bacteria between partners in just ten seconds. While many disease-causing agents can theoretically pass through this exchange, the actual risk is low for most healthy individuals. The risk depends heavily on the specific infectious agent, the viral or bacterial load present, and the immune status of the recipient. This discussion focuses on diseases where kissing is a known or likely route of transmission.
Viral Infections Spread Primarily Through Saliva
Infectious Mononucleosis, often nicknamed “the kissing disease,” is a prime example of a virus transmitted efficiently through shared saliva. The Epstein-Barr Virus (EBV) causes the vast majority of mono cases. EBV can remain in the saliva for months after initial infection, allowing for transmission even when the person feels well. While children often experience mild or asymptomatic infection, adolescents and adults typically develop the characteristic symptoms of fever, severe fatigue, sore throat, and swollen lymph nodes.
Cytomegalovirus (CMV) is another herpes virus commonly spread through saliva, making kissing a possible route of transmission. Like EBV, CMV infection is extremely common and usually causes no symptoms in healthy individuals, though it can cause a mono-like illness. Because it can be present in many body fluids, including saliva, close contact facilitates its spread.
Other respiratory viruses are also easily exchanged during close face-to-face contact, including those responsible for the common cold or influenza. Mumps, caused by a virus that infects the salivary glands, can also be transmitted via direct contact with infected saliva.
Infections Transmitted Through Contact with Open Sores
Herpes Simplex Virus Type 1 (HSV-1) is the most common virus transmitted through kissing that requires direct contact with the oral mucosa or a lesion. HSV-1 causes oral herpes, commonly known as cold sores or fever blisters, which are fluid-filled lesions on or around the lips and mouth. Transmission occurs most readily when an active sore is present and shedding the virus, though it can also happen during periods of asymptomatic viral shedding. The virus enters the body through small breaks in the skin or mucous membranes. Contact with an active sore should be avoided completely to prevent transmission of this lifelong viral infection.
Certain Sexually Transmitted Infections (STIs) are also relevant in this category because they require contact with an active sore or lesion, rather than just saliva. Syphilis, a bacterial infection, can be transmitted through kissing if an infectious sore, called a chancre, is present on the lips or inside the mouth. Human Papillomavirus (HPV), which can cause oral warts and certain cancers, is also transmissible through mouth-to-mouth contact, particularly if micro-abrasions or lesions are present. For these infections, the risk is tied directly to the presence of an active, contagious lesion.
Bacterial and Fungal Infections Spread Through Close Contact
Several non-viral pathogens can be spread through the close contact inherent in kissing, often circulating through respiratory droplets and throat secretions. Meningococcal disease, caused by the bacterium Neisseria meningitidis, is a serious infection that can lead to meningitis or blood poisoning. This bacterium colonizes the throat and can be transmitted through respiratory droplets and throat secretions, making close intimate contact, including deep kissing, a risk factor, especially among adolescents and young adults.
Strep throat, caused by Group A Streptococcus (GAS) bacteria, is highly contagious and spreads easily through saliva and respiratory droplets. The exchange of saliva during close contact is an effective way for the bacteria to spread from an infected person’s throat. Good oral hygiene practices can help limit the colonization and spread of these common oral bacteria.
Oral Candidiasis, commonly known as thrush, is a fungal infection caused by an overgrowth of the Candida yeast, which is naturally present in the mouth. The Candida fungus can be transmitted via saliva. Whether a recipient develops thrush depends on their individual risk factors, such as a compromised immune system, recent antibiotic use, or underlying health conditions.
Preventing Transmission
Taking practical steps can significantly minimize the risk of transmitting infections through kissing. Avoid all forms of close contact when experiencing symptoms of illness, such as fever, sore throat, or active respiratory symptoms, as this reduces the viral load being exchanged.
If a person has an active cold sore, blister, or any open lesion in or around the mouth, they should refrain from kissing until the lesion has completely healed and the skin has returned to normal. Maintaining excellent oral hygiene, including regular brushing and flossing, helps to reduce the presence of many bacterial pathogens in the mouth. Individuals should also avoid sharing items that come into contact with saliva, such as drinking glasses or eating utensils, as these can harbor and transfer infectious agents.