Matt. 13:33 Another parable spake he unto them; The kingdom of heaven is like unto leaven, which a woman took, and hid in three measures of meal, till the whole was leavened.

In one sentence, the Lord Jesus provided one of the most powerful, relevant, contemporary metaphors of the kingdom of heaven.  What is leaven?  Today, we call it yeast.  But whether you call it leaven or yeast, it is a bacteria. How do we define bacteria?  The definition of bacteria from vocabulary.com is:
Bacteria are microscopic living organisms, usually one-celled, that can be found everywhere. They can be dangerous, such as when they cause infection, or beneficial, as in the process of fermentation (such as in wine) and that of decomposition.” What is interesting about bacteria is that you don’t see it working, you see the EFFECTS of it working.  In other words, you don’t see the leaven working in the meal, or dough, you see the EFFECTS of the leaven in the meal.  And this is what happens with the kingdom of God in a person’s life.  You don’t see the kingdom at work in a person, you see the EFFECTS of the kingdom in a person.
But leaven has another quality that makes it like the kingdom of God.  Like all bacteria, leaven is a contagion, meaning it spreads by contact.  Here are two definitions from vocabulary.com for contagion:
“Any disease easily transmitted by contact.”
“The communication of an attitude or emotional state among a number of people.”
Vocabulary.com uses this example to describe the phenomenon of contagion:
“Have you ever noticed how when one person yawns, the people around him tend to do so as well? This phenomenon can be described as a contagion, the spreading of an emotional or mental state (in this case, fatigue).”
So contagions spread by contact.  Even in the example, the people affected by the yawning person were the ones around him, the ones who were close enough to see him.  Others, not close in proximity, may have seen or even heard him from a distance, but were not affected.  What then, does this mean for us today?  It means the gospel is still spread through close contact.  It isn’t spread by being seated in a large gathering or assembly once a week; it is spread in your day to day contact with your co-workers, with your classmates, with your neighbors, and with your friends.  The kingdom within you is supposed to be contagious; those around you don’t see the kingdom in you, they see the EFFECTS of the kingdom in you.  They see the change in you; they see the patience you are exhibiting; they see how you esteem others higher than yourselves.  These are all effects of the kingdom working its way through your life.  Are you contagious?