Modes

Easy: Easy mode is a starter mode where any symptoms mark infection by the pandemic, everybody obeys lockdown, and the virus cannot spread to or from robots that are isolating. Test results are immediate, contact tracing is instantaneous, and hospital beds are plenty and a guarantee of survival. There are no false test results, so anyone who's been tested is marked 'recovered' the moment their symptoms subside. It is not difficult to avoid deaths completely in Easy mode and very simple to keep costs to double digit numbers.

Medium: There are more robots on screen, meaning a higher rate of contact and other diseases show up, so you'll have to be more aggressive with your testing and can't rely on symptoms to guide you to the infected. Hospital beds are fewer so you'll have to ration them more carefully. Zero deaths is achievable though tougher and costs should be kept to the low triple digits, even though lower costs will be possible in many scenarios.

Hard: The disease is more infectious and robots travel faster and do not always obey lockdown. Hospital beds are limited so be vary cautious. This is the only mode that has antibody tests, so you can detect cases even if they've recovered and keep your endgame statistics clean. Double digit deaths and triple digit costs are common.

Pandemic: This mode tries to model the current crisis most closely. The biggest change is that the in-game stats no longer reflect the actual numbers, but only the results of testing. Tests also give false results on occasion and hospital beds are very limited, as they have been in many places around the world. The best way to avoid high death rates is to stop the spread. Look up the statistics on death rates in your country and try to do better in this mode. Four digit costs are common in this mode, and high triple digit costs mean you have done well.

Impossible: Models a worst-case scenario, where there are many robots moving at high speed and the disease is very infectious. People mostly don't obey lockdown rules, tests often give bad results, and a larger number of robots will exhibit critical symptoms if infected. It is very difficult to achieve optimistic results in this mode. I have found one strategy that works most of the time, but I'll leave that to you to figure out!