Being close enough to breathe air freshly expelled by an infected person can be enough to be at risk of resp. viruses carried in saliva aerosols and a kiss isn't necessary, surely? I don't know how far that applies to viruses commonly associated with developing ME.
It depends a lot on the particular virus, and also the phase of the infection. During the acute phase of a highly contagious viral infection like say influenza A or norovirus, just ordinary social contact is usually enough to transmit the virus, as global influenza pandemics illustrate.
However, for viral infections that persist in the body (such as herpesviruses and coxsackievirus B), once the acute phase is over and the virus goes into latency, the propensity to transmission is much lower, as there are far fewer viral particles in the saliva.
In the case of Epstein-Barr virus, even though 90% of the adult population have this virus in their body (usually in a latent state), most children and younger teenagers are not infected with EBV (in the developed world at least), despite the fact they are surrounded by adults who are EBV-infected. It's not until the late teens or early 20s that EBV is typically first picked up, sometimes by kissing a girlfriend or boyfriend (hence the name "kissing disease" for glandular fever / mononucleosis).
Some years ago when I first read the studies finding a higher prevalence of child sexual abuse among ME/CFS patients, my first thought was that this might be explained by the transmission of viruses to the poor abused child.
That's not to rule out the possibility that childhood physiological trauma may permanently affect things like cortisol levels and immunity (
this study found differences in cortisol in adults abused as a child). But the way I see it, in these studies which try to examine the long-term health effects of such psychological trauma, you cannot control for the possibility that child sexual abuse might also involve the transmission of infections to a young child, as well as cause a psychological trauma.