So, it looks like most asymptomatic infections later become infectious. Basically every person who gets symptoms had a multi-day asymptomatic infectious period they went through. SARS-CoV-2 does a better job than most of avoiding the automatic immune system because its large genome lets it encode a bunch of proteins that aren't for making new viruses but rather screwing with the immune system in ways that let it reach high numbers before the immune system catches on. Every virus that can actually make you sick does this but SARS-CoV-2 seems better than most. But frequently you're able to fight off the infection at a later point before the part of the automatic immune system reaction that makes you feel sick kicks into play but after you're got a chance to infect other people.

The normal course of the automatic immune system wiping out an invading pathogen without you noticing is that it happens immediately without you noticing and you never get a chance to infect anyone. But if that doesn't happen for COVID-19 or influenza or most things there'll be a time period after infection but before you notice anything where you're infectious. For COVID-19 this period is particularly infectious compared to the flu or SARS-1 or most things. It might be that flu and other coronaviruses tend to people who are infected enough to transmit the virus but never go on to develop symptoms. I don't know in that case.

WHO have maintained for months that asymptomatic patients are not infectious.

Until very recently they still said

"The risk of catching COVID-19 from someone with no symptoms at all is very low."

They now say "Some reports have indicated that people with no symptoms can transmit the virus. It is not yet known how often it happens. WHO is assessing ongoing research on the topic and will continue to share updated findings."

Yes, the WHO was incorrectly relying upon information provided by China. Since then it has become an accepted fact that asymptomatic transmission of SARS-CoV-2 is not only occurring, but is one of the primary transmission vectors.

https://www.nejm.org/doi/full/10.1056/NEJMoa2008457