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May 16

Why does viral load matter?

If viruses reproduce exponentially, one might naively expect a low viral load to simply cause a slight delay in the onset of an illness. Even one virus turns into a large number in a short amount of time. So how is it possible that any viral infection could have a severity that depends on whether you start off with one virus particle or 100,000 virus particles, given that one in theory should quickly turn into 100,000? 2600:8806:3400:3DB:4970:60BC:F1C1:34BF (talk) 15:30, 16 May 2020 (UTC)Nightvid[reply]

Whereas if you started with 100,000 particles, you'd shortly end up with 10,000,000,000 particles. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:22, 16 May 2020 (UTC)[reply]

If your farmlands get infested with breeding rabbit couples, does it matter whether it is one pair or 100,000 pairs? In the first case you might be able to control the situation by hiring an exterminator specializing in rabbits. In the second case, even if you employ all terminators from the whole State, the problem will overwhelm them. --Lambiam 17:46, 16 May 2020 (UTC)[reply]

We have an article on viral load, but unfortunately do not have one on initial viral load which is what I assume you are asking about, most likely with respect to COVID-19. I believe the answer is that it takes some time after initial antigen exposure for your immune response to ramp up, and a higher initial viral load gives the virus a head start.

So if you are infected with a very low initial vital load, your immune response may be in full swing before the viral load is sufficiently high to cause illness, resulting in an asymptomatic case. But with a high initial viral load, not only will the threshold have been exceeded for even longer before the effective immune response, but the viral load will be even higher when it does takes hold, leading to a sever illness.

I'm sure that explanation is over simplified, but hopefully not to the point of being useless. I didn't find any references of the mechanism from a cursory search, but I'll keep looking. -- ToE 01:15, 17 May 2020 (UTC)[reply]

Here something, not much, about Minimal_infective_dose. Some more relevant points: after infection and before replication begins the virus must find some susceptible cells, enter them and start replication. This can take hours or even days, during which time the virus is vulnerable. Replication time is also not zero: it varies from a minimum of 13 minutes for some fagi to 1.2 days for HIV and at least 30 hours for one hepatitis B virus. All together this means that at some low viral load immune response can have time enough to whipe out all attackers before they become dangerous, just as ToE said. A figure one meets often is 100 000 as a minimal infective dose 2003:F5:6F08:8200:351A:656A:496C:6A58 (talk) 12:47, 17 May 2020 (UTC) Marco PB[reply]