There are a variety of vicious disasters that are part of the Anno 1404 game to add extra difficulty. Disasters can be turned on or off individually in Continuous Game. All disasters will receive a verbal warning and notification.
Fire: can break out randomly in the city. Can slowly spread. Even if fire is turned off, fires can still be started by mobs, arson spies, Thunderstorms, Volcanoes, Tornadoes and building damage. Peasants that are happy will work hard together to carry water-buckets to the fire and can usually put out a fire themselves. Fire station can be built to combat fires and roads can be upgraded to stone roads to allow quicker fire control. Fire damage can also be repaired by Carpenter's house, so there are many countermeasures to fire so it is the least threatening of the disasters.
Thunderstorm: thunderstorms loom over occidental islands (green islands) for several minutes. During a thunderstorm a few weaker buildings may be destroyed like growing fields if they get struck by lightning which can do approx 200 damage and set buildings on fire. The Thunderstorm is restricted to not often fall on main island and lightning usually does damage approx. 3 times. Also thunderstorms greatly reduce a player's visibility for the island. Thunderstorms are a large thick dark gray cloud, heavy rain and also have a water droplet camera lens effect, making visibility very low. The countermeasures are the same as fire, emergency service buildings Fire station and Carpenter's house.
Sandstorm: randomly covers most of an oriental island (desert islands) in a sandstorm for several minutes. The effect depends whether it effects the production or the population directly.
Sandstorms, if affecting the production, can be really damaging to an oriental island economy because Noria will cease to provide fertility while continuing to run at full maintenance and production will be halted. As it also may affect the food production, the population may also protest against the insufficient needs if the supplies run out. The best way to counteract a Sandstorm is to stockpile, import and move resources around in warehouses as needed to prevent your Orient population from dying off from starvation and thirst.
If the sandstorm directly affects the housings, then the population will become displeased and irritated, and move out until the minimum of inhabitants is reached (Bazaars and other public buildings will continue to work however, so they will not start an uprising just because they can't have access to their public buildings anymore).
Plague: adds a green skull above a house and spreads randomly through houses slowly lowering population, downgrading houses if it reaches the minimum threshold. If enough population is affected then foreigners will not trade with the island until the lengthy plague is cured. Can be countered with Surgery building. Beggars increase the chance of Plague if an alms house isn't provided for them to live in.
Volcano: Volcanoes provide siginificant mutable resources, but on occasion may awake, become active or even erupt, causing debris to fall within range (all buildings, units and ships hit by debris will be destroyed instantly, save for monuments, military buildings and other high HP-buildings) and possibly leaking lava in a short radius, continuously damaging the surroundings and affecting the flora of the island. Landscape that is affected by the lava is temporarily unavailable to be built on until the enviromental effects fade away. While the Fire station and Carpenter's house can help with mitigating the aftermath damages caused by the volcano, they are however helpless should a volcano continue to erupt.
Tornado: the king of the disasters. The tornado should only be experienced by those who want to be punished brutally and don't mind rebuilding half their island. The only countermeasure are having emergency service buildings on hand to deal with the fire aftermath such as Fire station and Carpenter's house. However the Tornado mostly just sucks up entire houses but can also catch nearby buildings on fire. Visual reference[1].
Spy: added content with Venice expansion and acts as a disaster. A spy can be stopped by clicking on them in your main cities. Spy wear a medieval bird beak mask (called a Plague doctor costume in real history[1]). A spy can cause various disaster effects and should be stopped by manually clicking on them or by building a Base of Operations. More info on Espionage.
TIPS
Lord Richard Northburgh has an attainment called Court Architecture which can help buildings survive disasters by giving them 10/15/20% extra HP.
Utilize emergency service buildings such as Fire station, Carpenter's house & Surgery. Always leave space near the center of your city for these buildings because they have a radius that they effect. If you want to save on the gold maintenance cost of the buildings, wait until the disaster occurs first, then build emergency service where it is needed.