Here are some ways that diseases pass from person to person:
One person exhales the disease, another person breathes it in.
One person sheds the disease in excrement; another person gets it on their hands or on their food, and ingests it. (yuck)
One person gets it in their blood or body fluids; another person contacts those fluids, via a blood transfusion, a shared hypo needle, or unprotected sex.
A bloodsucking insect bites a diseased person, and gets infected; it then bites another person and infects them.
People, however, will try to prevent you from spreading. They will try to limit the range of insects (arthropods) that carry disease; they will be careful when coming into contact with body fluids; they will purify their water and exercise good personal hygiene to prevent fecal-oral transmission of disease.
Airborne diseases, however, prove to be very hard for humans to control. Historically, airborne diseases such as the flu have only been limited by the time it takes to travel from place to place... and by quarantines. But today, with jet travel, these strategies rarely work.
Good job! Especially if you are an airborne disease, you have found a way to pass readily from person to person within the human population. Now you reach the third barrier: how do you actually get inside a person's body, to find the cells you need to reproduce? Any ideas?
Jot them down, then continue!