Saliva can be a carrier of both bacteria and viruses, which can be transmitted to other humans and cause infection. If you think that you may have had exposure to someone who is ill, such as a cold, gastroenteritis or the flu, and have developed symptoms yourself, then I recommend that you see your primary care doctor
for evaluation and testing if necessary. The main sources of infection that are transmitted by saliva include bacteria and viruses that infect the gastrointestinal and respiratory tracts. These can include the paramyxoviruses that cause influenza, which is a systemic constellation of symptoms that starts in the upper respiratory tract, adenoviruses that cause the common cold, as well as multiple bacteria types that can cause sinusitis, pharyngitis (the most common type is strep throat), and pneumonia
. Generally, humans put their hands to their mouths and noses where these microbes can be found several times per day, and you are correct that hand shakes or touching a public door handle, etc. are potential mechanisms by which to come into contact with saliva based microbes from other people. This is why certain public health
programs are largely directed at regular hand washing and sanitation, as well as not coughing or sneezing into the palm of one's hand. Finally, gum under the table is often less of a risk because it has likely been there for several days to months or years and the majority of saliva viruses and bacteria associated with the gum have expired or are no longer infectious. Again, please speak with your doctor if you develop any symptoms.