Not everyone is allergic to bed bugs. Everyone reacts to their bites differently - that's why it's often so hard to even tell what is biting you. Some people get rashes, some people have minor bites, and some people don't see anything on their skin whatsoever. In fact, it's actually pretty common for people who are bitten by bed bugs to not see anything at all on their skin. So one person might happily sleep with them and never notice - until their unlucky spouse, who is more allergic, gets in the same bed and finds themselves covered in bumps in the morning.
Early infestations can be difficult to detect and the symptoms of bed bugs might often be detected before the bugs themselves. Typical symptoms of a bed bug infestation may include the discovery of fecal spots, crushed bed bugs and/or bloodstains on the sheets, the finding of cast skins that have been discarded, the development of itchy welts from their bites (in those that react), etc.
Use of a crevice tool to reach into small spaces as well as the use of a magnifying glass and/or flashlight can assist in the successful detection of bed bugs and their symptoms. A crevice tool, however, can be improvised but - very importantly - should consist of a non-conductive material such as plastic or wood if it is to be used to probe walls, baseboards, and/or electronics, etc.
Not everyone is allergic to bed bugs. Everyone reacts to their bites differently - that's why it's often so hard to even tell what is biting you. Some people get rashes, some people have minor bites, and some people don't see anything on their skin whatsoever. In fact, it's actually pretty common for people who are bitten by bed bugs to not see anything at all on their skin. So one person might happily sleep with them and never notice - until their unlucky spouse, who is more allergic, gets in the same bed and finds themselves covered in bumps in the morning.
In short, just because only one person in your house has bites doesn't mean you don't have bed bugs. And it doesn't mean they have some kind of vendetta against you, as opposed to your family members. It just means you live with someone who happens not to have skin reactions to their bites.
