Not a lame question at all! It’s astute. Viruses differ in the amount of virions necessary to cause infection because the immune system sometimes does have the ability to fight a particular virus before you get symptoms.

In the case of HIV, HIV positive people on Prep or other drug therapy AND with a very very low titer have been shown to not transmit the virus to intimate partners.

Measles requires a very low amount of virus to cause illness while a fairly large dose of rhinovirus can be handled by the immune system without causing symptoms.

Viruses and our immune systems are fascinating and I could go on all day! Lol