Because they are more vulnerable to any infection.
Anyone can be infected by T. gondii, but usually only those individuals with weakened immune systems (immunocompromised) develop symptoms of the disease. For them, toxoplasmosis can be severe, debilitating, and fatal.
The prognosis for acquired toxoplasmosis in adults with strong immune systems is excellent. The disease often disappears by itself after several weeks.
Weakened immune systems.
An opportunistic infection (OI) is an infection that occurs only when the host's defense systems (ie, the immune system) are weakened. The common characteristic of all agents that cause opportunistic infections is that they rarely affect individuals with healthy immune systems. For example, the disease AIDS (acquired immunodeficiency syndrome) is a classic cause of immune system depression and is associated with many opportunistic infections, including toxoplasmosis, pneumocystis pneumonia (PCP), and others.
Their immune systems have weakened with age.
People who have toxemia are often susceptible to infection because their immune systems are weakened
Most individuals who contract toxoplasmosis do not require treatment because their immune systems are able to control the disease. Symptoms are not usually present. Mild symptoms may be relieved by taking over-the-counter medications
Malnourished children may be short for their age, thin, listless, and have weakened immune systems.
Their immune systems were not used to diseases, as there weren't a lot of diseases at the time.
Oral candidiasis typically occurs in people with abnormal immune systems.
When Aspergillus does cause disease, however, it usually occurs in those individuals with weakened immune systems
Once infected, an individual is immune to reinfection.