An all-too-common issue that players of Cities: Skylines run into is a wave of sickness that spreads across residential neighborhoods and leads to a massive amount of death and abandoned buildings. Abandoned buildings drag down the land value and don't pay any municipal taxes, so this process can quickly lead to a game-ending deficit.
This article will attempt to help new players by pointing out the main causes of sickness and mass death that can cause a city to depopulate. Most of these issues have straightforward fixes, so once players know what to do they should be able to avoid the same situation in the future.
Contaminated Water
The number one issue that leads to mass sickness and death in Cities: Skylines is a contaminated water supply. If water pumps draw up brown, polluted water, or if water towers sit on polluted land, then the citizens who drink this water will start to get very sick very quickly.
To avoid this, make sure that all sewage pipes and all water treatment plants are located downstream from every water pump. Players can see the direction and strength of water currents by clicking on one of these buildings and then zooming in on the water surface. This will display arrows on the water, with the direction indicating where the water flows and the size showing the current's strength.
Players can also see the water current by placing down a sewage pipe and letting it run for a while. The pipe will dump pollution into the water, and this pollution will spread out based on the way the water is moving. Pollution is also visible in the pollution info view, and the icon for this is a purple barrel.
Something else to keep in mind is that water pollution will spread a little upstream from sewage pipes and water treatment plants, so players should keep water pumps as far away from sewage pipes as possible. Another issue is that water pollution will spread in every direction if there is no current, so players should never dump sewage and pump water from the same lake. Even if the pipes are on the far side of the lake from the pumps, polluted water will eventually make its way over. This is why every standard map in the game has running water in the starting tile.
Water towers work differently, because they draw water from underground wells instead of lakes or rivers. This means they don't worry about water pollution, but they do worry about ground pollution. Water towers can go anywhere on the map since they only need electricity to work, so be sure to place them far away from industrial zones, garbage dumps, and any other source of pollution.
Backed-Up Sewage
Another factor that can harm citizen health is backed-up sewage. However, this is a big enough problem that citizens will probably abandon the affected buildings before their health drops very far.
Keep in mind that the game tracks water and sewage separately, and players must have enough of both types of building to keep their cities running. If players have enough buildings, check whether all of them have water and power connections and raise the budget if it's below 100 percent.
Bad Healthcare Access
Another reason why citizens might get sick is because they don't have good access to healthcare. In the base game, medical clinics and hospitals are the only buildings that contribute to healthcare availability, and the other buildings in the healthcare tab are for other types of care. To keep average citizen health up and minimize the number of sick citizens, players should build plenty of clinics and hospitals so that every zoned building (not just residences!) Has good access to healthcare.
Players may also run into issues even when they have plenty of healthcare buildings. This can happen if traffic congestion gets bad, because healthcare buildings need to send out ambulances to collect sick citizens and bring them in. If an ambulance can't reach a citizen in time, they'll die. Fortunately, healthcare buildings also have the passive effect of making citizens get sick less often.
Ground Pollution
Even if the water supply is clean, pollution can still make citizens sick. If ground pollution overlaps the land that a residential building stands on, then the health of the citizens living there will slowly drop. Healthcare facilities can help counter this effect, but the best solution is to build industrial and residential zones far apart, and to make sure all the garbage facilities and power plants are also far from residences.
Noise Pollution
Noise pollution isn't nearly as dangerous as water and ground pollution, but it can still make citizens sick if it gets too high in a residential neighborhood. In general, industrial and commercial zones create noise pollution while offices and residential zones want to avoid it. Unique buildings, power and water buildings, transit stations, and heavy traffic also create noise pollution.
To keep noise pollution away from residences, players should stick with narrow roads in residential neighborhoods (preferably with noise-reducing decorations) and upgrade highways that pass nearby with sound barriers. Players should also avoid mixing residential zones with anything other than office zones and quiet service buildings.
Cities: Skylines is available now on PC, PS4, PS5, Switch, Xbox One, and Xbox Series X/S.