51.

1. psychological
2. pathological
3. physiological
4. immunological

52.

1. chronically
2. acutely
3. unfortunately
4. unexpectedly

53.

1. by means of
2. by way of
3. by contact with
4. by virtue of

54.

1. infecting
2. being infected
3. infecting with
4. getting infected with

55.

1. unsterilized
2. contaminated
3. infected
4. polluted

56.

1. similar to
2. unlike
3. instead of
4. compared with

57.

1. enjoying
2. employing
3. sharing
4. exchanging

58.

1. so
2. something else
3. whatever
4. the like

59.

1. vaccinated
2. evacuated
3. hospitalized
4. neutralized

60.

1. medications
2. tablets
3. doses
4. dosages

(Reference keys.)