That idea is not true and in fact is extremely dangerous thinking. One example is HIV. It isn't a matter of having it or not having it, the viral load makes a big difference as to what symptoms someone may exhibit as well as how likely they are to infect someone else. If your viral load is low enough then the chances of infecting other people can get low enough to be considered non-existant. On the other hand if you expose yourself to more sources of HIV, it could increase your viral load and increase symptoms and complications associated with the virus.