By the time a search engine flags a site as dangerous, the infection has almost always been there a while, it did not just appear. There are earlier signs, often subtle, that show up weeks before the problem becomes undeniable. Recognising them in time is the difference between an afternoon's cleanup and a crisis with the site publicly marked as unsafe.
Why it is worth learning to tell them apart from an ordinary bug
Not every strange behaviour is an infection. A site can run slow because of a badly written plugin, can have a visual glitch from an outdated theme, can have a form fail from a configuration issue. The key to not overreacting nor ignoring something real lies in a specific pattern: infection signs tend to be unexpected, to show up where you did not touch anything, and to involve content or behaviour nobody on the team generated.
The most common early signs
Unexpected redirects. The site, or some of its pages, sends visitors to a different address nobody configured. Sometimes it only happens for certain visitors (those arriving from a search engine, for instance) and not when the site owner checks it directly, which makes it easy to miss for a while.
Content nobody on the team published. New pages on topics completely unrelated to the site, often linked to products with nothing to do with the business. It is one of the most common techniques: using a legitimate site to rank unrelated content by borrowing its reputation.
Administrator users you do not recognise. Checking the list of users with admin permissions and finding one nobody on the team created is one of the clearest, least ambiguous signs there is: there is no innocent explanation for that.
The site sends mail you never generated. Bounces for messages you never sent, or complaints that your domain is sending spam, indicate something is using the server to send mail without your knowledge.
Changes to files you never edited. If the hosting shows file modification dates, and files show up modified on dates when nobody on the team touched anything, that is a direct sign of unauthorised activity.
The browser or the hosting's antivirus starts flagging something. If you have malware scanning included with your hosting, pay attention to any alert, even one that looks minor or isolated.
Outbound links you never added. Checking the pages' code, links to external sites show up that nobody added, generally hidden so they are not visible at a glance in the design.
What to do the moment one of these signs appears
Do not panic, but do not ignore it hoping it resolves itself either. The sooner you act, the simpler the cleanup usually is.
Run a malware scan if your hosting includes one, for a more concrete diagnosis than mere suspicion.
Change passwords for the panel, the site administrator, the database and FTP access, in case the entry point was a compromised credential.
Check the full list of administrator users and remove any you do not recognise.
If you have backups from before the signs started, restoring from there is usually more reliable than trying to clean file by file, because there is no guarantee of having found every backdoor by hand.
Only after cleaning, update everything: core, plugins, theme. The update alone does not repair an already compromised site, but it does close the door they probably came in through.
Why acting before Google's warning matters so much
Once a search engine flags the site as dangerous, the impact goes far beyond the technical cleanup: the visible warning scares off any visitor who sees it, and rebuilding trust (both the search engine's and that of anyone who once saw that warning) takes longer than resolving the infection itself. Acting at the early-sign stage, before it reaches that point, is the difference between an incident almost nobody notices and one that visibly damages the site's reputation.
A short review worth doing regularly
You do not need to become a security expert to catch these signs in time. A ten-minute review every so often (checking the administrator user list, browsing a handful of pages looking for anything odd, checking for any unexpected mail bounce) catches most infections at their early stage, well before they turn into a problem visible to everyone. It is a small routine compared with the time a cleanup demands once an infection has already escalated.
Adding it to the same routine where you already check that backups are running is a simple way to make sure it actually happens, instead of staying a good intention that never turns into a habit.
And if any of these signs shows up right after installing a new plugin or theme, suspect that first: it is by far the most common entry point, and checking that specific piece is usually faster than starting the investigation from scratch.
None of this replaces a real security scan once something looks genuinely wrong, but it does mean you reach that point with a head start instead of starting completely blind, which is usually what separates a quick, contained cleanup from a much longer and more expensive one.
A site that catches an infection early rarely makes the news, in a good way. That is precisely the outcome worth aiming for.