Meaning:
“幻” is short for “幻想,” which means “have illusions,” “阳” here means “COVID positive,” and “症” means “disease.” Coined by Chinese netizens, this term refers to a kind of hypochondria related to COVID infection. Some who see many of their friends and colleagues becoming infected with COVID may feel under stress that they themselves have COVID symptoms too, although they are not actually infected.
Example:
A: 丽丽今天请假没来上班，是阳了吗？
Lìlì jīntiān qǐngjià méi lái shàngbān， shì yáng le ma？
Li Li is on sick leave today. Is she infected with COVID?
B: 她说自己不舒服，但测了几次抗原还是阴性。
Tā shuō zìjǐ bù shūfu， dàn cè le jǐcì kàngyuán háishì yīnxìng。
She says she feels unwell, but has negative antigen test results.
A: 大概是幻阳症吧。
Dàgài shì huànyángzhèng ba。
She could be hypochondriac.
B: 也可能是抗原检测不准。
Yě kěnéng shì kàngyuán jiǎncè bù zhǔn。
Or it could be the antigen tests she took were inaccurate.