As Madison and her group travel further down the California shoreline, they discover that Mexico closed the border crossing from the United States, making land travel impossible. As they get closer to Rosarito, they encounter the Mexican Navy, indicating that there is still a large military presence operating near Mexico's borders.
Though Mexico has been largely overrun, it does seem to be functioning slightly better than the US, as a number of neighbourhoods in the city of Tiajuana have become secure safe havens and there is also a large trading post in a stadium near the border where many people from different Mexican survivor enclaves come together to trade.
It was mentioned by George Geary that all of the west coast cities of America got bombed. During his conversation with Travis, he states that Mexico has sealed its borders with the United States.
During his search on Strand's boat, Daniel Salazar found maps of the western coast of Mexico along with a loaded MP5 sub-machine gun.
It is revealed that Mexico is Strand's true destination. He states that he has a well-supplied, well-defended safehouse near Rosarito, Baja California.
The aforementioned house is seen during flashbacks in this episode. It is the home of Thomas Abigail and his family and it appears to be in a relatively secluded part of the country.
Outside of flashback, this is the first time we see Mexico. It is not as safe as people believe. A priest and his congregation are poisoned and later reanimated. However, elements of the Mexican Military, mainly the Mexican Navy are still active and patrolling coastal waters to stop refugees entering- or, at least, to stop those refugees who can't pay and the infected from entering.
When Celia Flores is talking to Daniel Salazar about the walkers locked in the wine cellar, she says she is keeping them safe, because outside they would be "hunted" and "killed" like they were "monsters" and that they are family. Since most of the staff and their families seem to live on the premises, the implication is that they are aware of other walkers who were killed outside the safe house and that the outbreak may not be limited to Baja. The condition of the country at large is ambiguous but it is possible that the outbreak hasn't reached epidemic levels yet.
However, the trailer for Season 2b of FTWD seems to suggest that the infection HAS reached epidemic levels in Mexico and that there have been outbreaks for some time. The evidence for this includes the decomposition of some of the walkers and the sheer number of undead.
In addition, when Strand was asked why the members of the flotilla didn't pursue the Abigail or sink it after the exchange that killed Miguel and Vazquez, Strand suggests that they (the military) believe that whatever was on land would kill them. This may indicate that the scale of the pandemic in Mexico is greater than previously thought, and thereby explain the apparent lack of military operations in the area.
Therefore, it can be assumed that coinciding with the outbreak in the United States, Mexico (or at least the state of Baja California) experienced similar circumstances and became overrun within 3 weeks.
Travis' search for Chris takes him to the edges of the Abigail Estate, where he finds free roaming infected, and small houses where some families connected to the Abigails still live. When the estate is destroyed by the actions of Daniel, everyone who survives flees.
Nick spends the night in one of the small houses near the Abigail property in Valle de Guadalupe. The homeowners were already dead. Sofia relates to him that there has been talk of a safety to the north, in the city of Tijuana, Baja California. She decides to head south, in search of Juan's father. Nick spends the next several days travelling through Mexico. The roads are cluttered with abandoned cars, small to moderate size hordes of infected roam the countryside, and outlaws patrol the new lawless world. Most of Baja California is abandoned and overrun, however, in and around the city of Tijuana, survivors have created walled communities which makes it a relatively safe place to be.