1. **Introduction of the game**
Plague Inc. is a unique mix of high strategy and terrifyingly realistic simulation game developed by Ndemic Creations. In this game, players are tasked with creating and evolving a pathogen in an effort to annihilate the human population with a deadly plague. The game combines strategy and terrifyingly complex variables, requiring players to plan their pathogen's spread meticulously.

2. **All characters and levels of the game**
The game does not have characters in a traditional sense, but it features various pathogens as the central "characters" that the player evolves and adapts. These include bacteria, viruses, fungi, parasites, prions, nano-viruses, and bio-weapons, each with unique traits and challenges. Additionally, there are special plague types, such as the Neurax Worm, the Necroa Virus (zombie-like plague), and the Simian Flu. As for levels, the game presents an increasing difficulty with scenarios that challenge the player to spread their plague in different environmental conditions and against varying levels of global defense and research.

3. **What attracts users most to the game**
Players are often drawn to Plague Inc. for its darkly realistic gameplay, strategic depth, and the morbidly satisfying challenge of trying to outsmart humanity's defense mechanisms and responses to disease outbreaks. The game's simulation of epidemic spread is complex but accessible, making it appealing to both casual players and those who appreciate detailed management and strategy games.

4. **Why it is popular**
Plague Inc. has gained popularity due to its original premise, thought-provoking gameplay, and timely subject matter. Its popularity spiked, particularly during real-world events that brought the threat of global pandemics to the forefront of public consciousness. The game also offers a sense of dark humor and the ability to indulge in 'what-if' scenarios which have resonated with a vast audience.

5. **When the game was released and how many versions are currently available**
Plague Inc. was initially released on May 26, 2012, for iOS devices, and it subsequently spread to Android, Windows, and consoles. There are several expansions and updates to the base game that have been released over the years, including Plague Inc: Evolved, which is a more comprehensive version that includes all prior updates and additional content.

6. **The most difficult part of the game**
The most difficult part of Plague Inc. is often adapting to the increasingly effective countermeasures deployed by humans, including countries shutting down their borders, developing cures, and other global responses that can halt the spread of your disease. Mastering the game requires a deep understanding of how different pathogens behave and evolve in response to these challenges.

7. **Strengths and weaknesses of the game**
Strengths of the game include its original concept, depth of gameplay, and educational value in illustrating how diseases spread and can be contained. Weaknesses might include a steep learning curve for new players and potential insensitivity given its theme, especially amidst real-world outbreaks.

8. **What other games have been developed by the same developer**
Ndemic Creations, the developer of Plague Inc., has also developed "Rebel Inc.," a political/military strategic simulation game where the player attempts to stabilize a country on the brink of collapse, and "The Cure," an expansion for Plague Inc. that flips the script where players work to manage the outbreak and develop a cure.