asymptomatic (not even sniffles), through mild cold to mild flu to severe flu to ARDS (where your alveoli fill with liquid and you suffocate). Many avoid mentioning "flu" because they want to avoid the naive take-away "it's like the flu." Many cases are like the flu; the disease is not like the flu.
Another reason to avoid saying "flu" is that for many muscle aches seem to be a standard symptom of influenza, but I haven't seen that reported for COVID-19. (On the other hand, in 60 years I've only ever had muscle aches with the flu once.) Note that the prevalence of the flu in January and February served as a good mask for covid-19, and in some places they went back and checked for covid-19 in the remains of old flu test samples that had came back negative and found more covid-19 positives.
Pre-existing immunity isn't the reason for the symptom range, so far as I've seen reported, so much as how the immune system responds. Option 1 is "normally" (asymptomatic or mild symptoms) or "panic" (severe flu to death). Since the two options aren't exactly narrowly prescribed, that gives you the full range, nothing to very severe.
For some reason, older people seem to have immune systems that more often don't respond properly. It really kicks in after age 60, and is worse for those over 70.
This is emphatically *not* the same thing as complications from pre-existing conditions, such as compromised immune systems or high blood pressure. Those trigger some pretty lethal consequences, and then if the health care infrastructure isn't there to help the symptoms you can get a pretty harsh spike in death rates.
Notice that the last two paragraphs help explain much of the variance in death rates. Even the Chinese "there are two strains of COVID-19" might be true, but also says that the known infrastructure failings in Hubei province had no effect. Or, perhaps, the infrastructure failings in Hubei account for some of the variance between the two strains. In the US, the data from Washington State, where nursing homes accounted for a lot of the early deaths, also skews things--if 50% of the population you know is infected is in the highest risk group it doesn't meet the requirements for the usual interpretation of the data, which is that the infection spread in a largely random way.
(But if you must look at the bright side of things, just imagine the effect of a 20% mortality for those over 70 years of age on the viability of the social security system and killing off all those nasty boomers who stand in the way of true American values. Full discloser: I'm a boomer and that's writ incredibly sarcastic.)
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