Laura,
It is not necessarily easy for viruses etc to pass the BBB. The ability depends on two properties, the molecular size of the toxin, and its fat solubility. Then what most people forget is the BBB permeability is not constant. it can be altered, stress opens it and curcumin closes it. So if you say dopamine can't pass it, you have to derfine what the permeability is that you are talking about.
It is even more complicated than that. Levodopa is said to pass the BBB, whilst dopamine cannot pass. However, dopamine is a smaller molecule than Levodopa!! This is due to the greater fat solubility of levodpa.
Dopamine has a lower molecular weight than Levodopa since it is formed by Levodopa losing a molecule of carbon dioxcide.
Hope this helps and is not too heavy!
Ron