The ideal number of players is 4, but our sessions were played with 3 players. As part of the setup, each player randomly draws a card from the "Role" deck. Each role card endows the player with a different ability. For example, the Medic can cure all diseases in a city in just one action, the Scientist can eradicate a disease using only 4 cards of the same color instead of the required 5 cards, etc. Therefore, playing with 4 players is most beneficial as you will have 4 different abilities contributing to the players' efforts to save the world. The rules are pretty simple and can be found here.

We played three sessions or games. The first was in heroic level - 6 Epidemic cards in the Players Cards deck. It resulted in total failure, which we placed on the fact that 2 of the 3 players had never played Pandemic before.
When we lost that first game, the fifth Epidemic card had not been drawn yet so we decided to play the second game in normal level - 5 Epidemic cards in the Players Cards deck. We lost this game even quicker than the heroic level game, lmao!

The third game was also in normal level. However, we now had a good idea of why we were losing. During most of the third game, we did a good job of controlling the disease outbreaks. By the time the fifth Epidemic card was drawn from the Players Cards deck, we had already eradicated one of the 4 diseases. This was a relief in the sense that there were still a good amount of cards left in the Players Cards deck and, being the game in normal level, we knew there were no more Epidemic cards to be drawn from the deck. We were feeling very confident that we would save the world this time around.
We did a good job of containing this last epidemic and already had the resources to cure 2 of the 3 remaining diseases left when... the 6th Epidemic card came out! WTF?? Yep, while one of the players was fetching some munchies from the kitchen and the other was opening a second bottle of wine, the buddy shuffling the cards inserted the 6th Epidemic card into the Players Cards deck without anyone else noticing. In the end, that evening we failed humanity 3 times in a row, but the 3 of us became addicted to the game.

If you have the opportunity to play Pandemic, do not let it pass. In time, I may find a strategy that will turn the game into a boring one. Until then, Pandemic will be in my list of top 5 board games.