Illegal immigration is one of those topics on which almost everyone has an opinion -- maybe several. But when it comes to the facts, the number of people with a grasp on them is a lot smaller.

It's no wonder. The United States has one of the most active and complicated immigration systems in the world. For instance, a different number of visas -- needed to come to the United States -- are issued each year in various countries. And the wait times for those visas varies greatly.

There are at least 4.4. million people waiting. Those figures include 1.3 million in Mexico alone. That system is also paired with a visa lottery and a series of special visa programs reserved for artists, models, scientists, international business executives and other people with special and needed skills. Then there are the millions of undocumented immigrants already living inside the United States.

It's a lot. That's why there is a Part I and a Part II to this post. But we've tried to boil it down to the essentials for your next immigration-related debate.

How many immigrants live in the United States illegally and where do they come from?

The Pew Research Center put this number at 11.2 million people in 2012, down from a high of 12.2 million in 2007.

Most -- 5.8 million people -- hail from our nearest neighbor to the south, Mexico. Another 1.7 million come from various countries in Central America. About 1.4 million people come from an Asian country. South America is responsible for about 700,000 people. Europe and Canada contributed 600,000. And Africa and the Middle East, 400,000. In addition to this, about 550,000 arrived from the Caribbean.

Check out Pew's dynamic charts for more info on the countries of origin or for a read on the numbers in your state. But here's the national picture between 1990 and 2012.

Is illegal immigration an increasing problem?

Well, a look at the chart above pretty clearly says the answer, at least for the last few years, is no. But there are also other bits of federal data that suggest a more complicated answer.

There was an unquestionable surge in the number of unaccompanied minors and women with children who last year entered the United States and, in many cases, turned themselves in to border authorities. And the Coast Guard also reported an increase in the number of Cubans trying to enter the United States to take advantage of what they fear will be the last days of their special immigration options.

Still, illegal immigration is not a mushrooming problem. But it is a real one. Federal agents apprehended nearly 50,000 people trying to enter the United States without permission last year.

What's causing the declining illegal immigration rate?

This is one of those situations in which lots of hands want to take some credit.

Federal agencies have invested heavily in border patrol activities. Border patrol is even using drones to monitor the area. (The nation's auditor issued a report late last year on civil liberties issues in the border region due to drone use, in case that's a concern for you.) And border state leaders such as former Texas governor Rick Perry, a Republican seeking the 2016 Presidential nomination, have touted their state's contributions to enhanced border patrols. Perry said that federal officials are unaware or unwilling to acknowledge the situation on the ground.

Immigration also dipped during the recession when few jobs were available in the United States. But many of the unaccompanied minors and families that streamed into the United States in 2014 did so to reunite with family members already here or to escape some combination of terrible gang violence and economic instability at home.

So geopolitical forces -- including some U.S. policies -- are all in play.